I use both programs - Lightroom 3 and Photoshop CS5. What Tony has said is all correct and wouldn't argue with any of it, but you should know that the one area that I think Lightroom excels is in the Library module. Its keywording and "library" functionality is much better than that of Bridge (IMHO) which is the reason I use Lightroom (LR) primarily for most of my image processing. Its "library" functionality is also one of the reasons I upgraded from Photoshop Elements, after a while, the Organizer just didn't cut it - for me, at least.
One main area where they differ is in the ability to process an image at the pixel level, for example, the content-aware feature in CS5 is not even close to being available in LR, so if I need to do some editing at this level, then I move the image from LR into CS5 and edit it there. I find that, in a way, they complement each other, that is not to say that most if not all of the editing you do in Lightroom cannot be done in CS5, however there is a lot of editing in CS5 that cannot be done in LR. LR does not do things, to my knowledge, like overlays, it does not do masking and layering, things that CS5 does very well. I have no experience with the ability of Bridge to "keyword and index" images, as I do not use it, maybe someone who does will comment on this capability. As Tony has indicated, particularly with regard to learning curve, it is long and steep - I have an advantage because I used its less expensive cousin - Photoshop Elements for many years and its interface is very similar to that of Photoshop, although, as Tony says, Photoshop has very significant functionality, and, as I found out...much more capability than Elements - heck there are over 500 menu commands in Photoshop CS5 alone...
You have to remember that, in a way, they are two completely different pieces of software and that Photoshop, in all its versions, was originally intended for web developers and graphic artists and not for photographers, who grabbed onto it because, at the time, there was no other "software" like it that could do all the things it did for photographers. The other thing that LR has going for it, again, my opinion, is the very easy-to-use interface between LR and CS5/Photoshop, so it easy to index your images in LR and even do a bit of editing in LR, but if you need the pixel-level editing in CS5, for whatever reason, then you can "Edit in Photoshop" very easily.
The other thing that I like about the interface is that you can take panoramas, import the original photos into LR, do all your keywording, etc. in LR and then export them to CS5 to make the panoramas. You can do a similar thing with HDR images - export them from LR to HDR Pro in CS5 and have it construct the HDR image. Once you have the HDR image the way you like it in CS5, you can then send it back to LR and do some additional editing, if you require it, in LR itself.
As far as "Actions" go, I do not use them, because they have to be built in Photoshop and this, in my opinion, requires a very significant knowledge of Photoshop. (You would need to ask yourself whether you would use them or not and how and when and why, etc...) Even in Lightroom, I do not use presets that much, beause they are always someone else's idea of what an image should look like and very often not mine. I do use presets for importing and have found this to be a successful way of using them, but I am a hobbyist and am not in the business of selling my images, so I don't really care for all the special effects that one gets from presets, although I do understand why people would use them, so don't take this comment as knocking presets, there is a time and a place for everything, including presets and maybe I just need to explore them a bit more - don't know. I shoot mostly landscapes, nature, wildlife, macros and most presets do not lend themselves to my kind of photography - but enough about presets...
I would agree with Tony - read-up, ask around (maybe others will post thoughts here as well), download the trial, in fact you might want to look at one book, that I have - Photoshop CS5 and Lightroom 3, A Photographer's Handbook, by Stephen Laskevitch and published by Rockynook. It covers both LR and CS5, as well as Bridge and will give you some insight into the three products and how they can and cannot work together. HTH and good luck.
Cheers,
WesternGuy
P.S. I see you got your post about HDRs and panos up before my response...so you know...