Hi and welcome to POTN...
A short course on focal lengths for your camera...
8mm to 12mm are considered Ultra Wide Angle. You might find these useful for landscapes, architectural interiors and other purposes. Wide lenses tend to render deep depth of field, everything from near to far in acceptibly sharp focus. The wider a lens, the more it will have some distortion effects... think of humorous photos of people with really big noses and tiny ears, done by getting close with a wide angle lens. (Note: there are even wider lenses called "fisheye", which have even stronger distortion effects)
15mm to 22mm are wide angle lenses. Less distortion, but still pretty wide to moderately wide.
24mm to 35mm lenses are "normal" or "standard" lenses (actually this range covers slightly wide normal to slightly telephoto normal lenses). These give approx the same angle of view as you see with your eyes and are among the fastest (largest aperture).
40mm to 85mm are "short telephoto" lenses on your camera. Within this range are popular portrait lenses... long enough to render a nice perspective (little distortion effects), and if it has a large enough aperture, able to better blur down backgrounds... but not so long that you need gobs of working space or start to see other types of distortion effects. There are also some macro lenses in these focal lengths.
90mm to 135mm are "moderate telephoto" lenses on your camera. These give more "reach", more blurred down backgrounds, but are becoming increasingly more difficult to handhold for a steady shot. There are also more macro lenses in these focal lengths. These might be used as "long" portrait lenses... or for moderate reach shooting sports... among other things.
150mm to 200mm are "strong telephoto", while 300mm and up are often called "super telephoto". There are also distortion effects with telephoto lenses. The main thing is "compression" of perspective. This is a lot more subtle than wide angle distortions. Telephotos in these categories are popular, among other things, for sports photography, for wildlife photography, and more. Anyone photography small subjects at a distance, such as birds, might want a really long telephoto lens... 400mm and up. There's a saying among birders, in fact, that there's no such thing as a "long enough" lens!
There is nothing that can be done about wide angle or telephoto perspective distortions.... It's just a fact of life and they aren't necessarily a bad thing anyway, once you learn to use them to your advantage. So, just be aware of and work with them.
There's a lot more to lenses, though, than just the focal length. As you've already noted, there are f-stops to think about, among other things.
I'm sure you are aware, there are zooms that cover a range of focal lengths and primes that offer only a single focal length. Your 18-55 is a zoom and your 50/1.8 is a prime. Zooms are far more complex, more difficult for manufacturers to "correct" well for top image quality and tend to be bigger and more expensive, but can be versatile and convenient. Primes are simpler and often are more easily corrected for top image quality, tend to be smaller and often offer bigger maximum apertures.
Your 18-55 is a convenient "walk-around" zoom covering a slightly wide angle to short telephoto range of focal lengths, but has a limited and variable aperture (f3.5 to 5.6). It's also lower specification in the autofocus mechanism and general build quality. It's an inexpensive kit lens. There are more expensive zooms with a wider range of focal lengths (Canon 15-85) or with a larger maximum aperture (Canon 17-55/2.8). In both cases, they are not only more expensive, but also larger and heavier, but you might expect top image quality from them.
You won't find "one lens that does it all really well". That would be huge, heavy and super expensive. The whole idea of a DSLR is the ability to change lenses to meet your particular needs. And Canon offers the largest selection of lenses of any manufacturer (plus third party manufacturers such as Tamron, Tokina and Sigma make even more lenses to fit Canon).
It depends upon what you want to shoot, the lenses you might want to choose and use.
For portraits, I like to use smaller, less intimidating prime lenses that also can have larger apertures to blur down backgrounds more, since that's often something you have little control over unless shooting in a studio. My two main portrait lenses for crop camera (like yours) are 50/1.4 and 85/1.8. But at times I use wider lenses (28/1.8 and 20/2.8) for couples, groups or wider "environmental" portraits that show people in their workplace or home or whatever. Other times for portraiture I deliberately use longer lenses (135mm, 200mm, even 300mm), either to work from farther away... perhaps for more candid shots, or to very strongly blur down the background, or for some deliberate compression of perspective.
The aperture of a lens decides how much you can blur down the background or use other shallow depth of field techniques, though this is also influenced by focal length. Of course, larger apertures also allow shooting in lower light. You have to balance it, though, because at larger apertures often lenses become less sharp and large aperture designs might be more susceptible to other issues such as chromatic aberration (where different colors of light are focused at different distances) and flare that can reduce image contrast and color saturation or add ghostlike artifacts to the image.
The shape of the lens' aperture plays a big roll in the look of blurred backgrounds, too. Generally speaking, the more perfectly round it is, the better. So, a lens with more aperture blades usually renders a nicer background blur (some lenses also uses curved blades, to further enhance this). For example, your 50mm lens has a 5-blade aperture. The more expensive Canon 50/1.4 has 8-bladed aperture and Sigma 50/1.4 has 9-bladed, whiile the premium and pricey Canon 50/1.2L has 8-bladed with curved blades. All these offer nicer "bokeh" than the inexpensive 50/1.8. But for someone just starting out and only using it occasionally, the 50/1.8 is a great introduction to prime lenses, a real bargain that takes far better images than it should, considering it's price!
You will find out that all lens choices are about compromise... The laws of physics limit what optics can do fairly rigidly and there's no getting around them. You can find super wide ranging zooms that try to cover 18-270mm all in one... but do they do any of it all that well? Those types of lenses might be fine for travel, but have fairly small apertures that limit them to use in pretty good light. And, in a zoom f2.8 is the largest aperture you will find and those zooms will necessrily be more expensive, larger and heavier. A prime lens might offer f1.4, two full stops more light... but lacks the convenience and versatility of a zoom.
There are a number of other factors to consider when shopping for lenses.
For example, I try to stick with USM lenses. This type of autofocus drive is faster, quieter and more accurate than the less expensive micro motor drive. Sigma has their own verion - HSM. And I believe Tamron has recently introduced something similar - USD - on a few lenses. This is more important on longer focal length and macro lenses, where shallower depth of field makes focus precision more critical or I'm more likely to be shooting fast moving subjects (sports) and need the speed of focus. Also, wide angle lenses only need to move their focus group slightly to achieve focus... while a telephoto lens might have to move it a long way (and macro lenses often have to move the focus group the farthest, in order to cover everything from infinity to 1:1 magnification a couple inches in front of the lens). One non-USM lens I use without any problem at all is an ultrawide zoom for crop camera... But I try to stick with USM (or it's equivalent) on most others.
Build quality is another major factor... I look for a lens that's going to hold up over the long run with regular use. And, this includes sealing against dust or moisture. Few lenses are completely sealed (maybe underwater lenses, such as the Nikonos line), but some are better than others resisting dust and/or moisture getting inside.
One of the key reasons I switched to the Canon system years ago was Image Stabilization. Canon was the first to offer this (on their 35mm film camera lenses, at that time) and it was some years before anyone else followed Canon's lead. IS is very valuable to me on telephoto lenses. I look for it and pay extra to have it on lenses approx 100mm or longer. I'd hate to be without it on 200mm and longer lenses. It's nice to have, but not as essential to me on shorter focal lengths, say below 100mm.
Little stuff matters to me, too.. such as filter size (does the lens use an odd size that means buying and carrying around more filters?) or if the lens includes or can be optionally fitted with a tripod mounting ring (telephotos and macro lenses). Close focusing distance matters, too... though it can be modified pretty easily with accessories such as macro extension tubes, with little loss of image quality.
For a whole lot more info about lenses and the Canon line in particular, you might want to go to this link
and download Canon Lens Work III. It's a good resource and full of info about all the lenses Canon makes. The book is all there in PDF format... 11 separate files I believe. There's also a lot of info about lenses at the Canon Learning Center. And third party lens manufacturer websites might be helpful to you, too: Sigma, Tamron and Tokina
I also highly recommend anyone new to DSLRs get the book "Understanding Exposure" by Bryan Peterson. Even though this is largely about making good exposures with your camera, it also goes into detail about lens selection, among other things.
Finally, if you narrow it down to a lens or two that you are considering, search for the specific lens at the Lens Sample Photo Archive sub-forum here on POTN to see images made with it and read peoples' comments. There are other websites that put lenses through exhaustive testing and post their results. Plus there is discussion ad naseum on individual websites, here on POTN and elsewhere on other websites and forums. Do a Google search for any particular lens you are considering, to get more info than you are likely to ever need or want!
See if there are any active photography Meetup groups or clubs in your area and go out and shoot with other people, to learn and grow your photography skills. Take a class. Look for some other books (there are hundreds of books on photography!). All these can be useful to expand your knowledge about lenses and how to get the most out of them.
Lenses are probably the most important factor in photography... They "make" the image. The camera behind the lens just captures it and all of them are pretty equal in quality now, as far as camaras are concerned. You'll often hear "Glass first, cameras second" here on POTN... Personally I'd rather have an excellent lens on the most entry level camera, than a cheap lens on the top-of-the-line camera that Canon makes.
EDIT: the Tamron 17-50/2.8 non-VC is a popular "budget" fast, mid-range or "walk-around" zoom. It's generally thought to have very good image quality (compares well with the far more expensive Canon 17-55/2.8 IS). It might be a little slower focusing, it lacks USM or the equivalent. Note, too, that most people don't think the more expensive Tammy 17-50/2.8 VC is as sharp as the non-VC version.
If you have some faster prime lenses at key focal lengths... such as the 50/1.8 or 50/1.4, 28/1.8, Sigma 30/1.4, etc... you might not need all that fast (f2.8) a "walk-around" zoom. Instead, simply switch to the prime lens when needed for low light or greater background blur. For example, the Canon 15-85 offers similarly high image quality with a significantly wider range of focal lengths than a 17-50 or 17-55, yet is more compact, because it's an f3.5-5.6 lens.
Along with several wider and longer lenses, I use a 24-70/2.8 and 70-200/2.8 a lot... But when I have to hike a ways and want to lighten my load... and lighting is good... I'll sometimes leave the two big L-series lenses at home and use a 28-135 instead. This old lens design is actually capable of quite good images (plus it has both IS and USM) and there are so many on the used market it can be relatively inexpensive... Some don't care for this focal length range on a crop camera, but I find the focal length range very useful. However I do complement by having a wider lens available most of the time.
