To clarify a bit:
ETTL is the term for "Let the camera figure out how bright to make the flash." It works like this:
1) As you press the shutter button, the camera meters the scene.
2) The flash fires a very low-power "test shot" pre-flash, and the camera meters how the scene changes based on that test shot.
3) The camera compares the exposures in the no-flash and pre-flash shots, and determines how much the flash affects the scene.
4) The camera decides how powerful to make the flash in order to get an overall "correct" exposure
5) The camera takes the real photo with the flash firing at the level it computed in steps 1-4.
All of this happens in a teeny tiny fraction of a second. You never see the pre-flash fire because it all happens so magically fast.
With the pop-up flash, you kinda take it ETTL for granted.. you never even know it's working, because it's working all the time, to make average exposures all the time. Same thing with a ETTL-enabled hotshoe flash - it just magically works (well, most of the time) unless you tell it otherwise. By using a combination of exposure compensation (EC), and flash exposure compensation (FEC) you can tell the camera to adjust the exposure without the flash, and also tell the camera, in very general terms, how much more you want to add with the flash. If you leave both set to 0, the camera and flash will try to make an average exposure every time - which is sometimes good, sometimes bad. So learn to use EC and FEC to make better-than-average exposures.
However, when you attach a flash to the hotshoe, if it doesn't have ETTL, then it'll be up to you to figure out how powerful to make the flash for each and every shot. This isn't really feasible for a novice, and it's not really fun for even an experienced person who's just walking around doing snapshots. So ETTL is kinda a must-have for a first/only hotshoe flash, or a walkabout flash where you don't want to have to futz with the flash power for every shot.
The 430EXII is a great choice. I use it as my primary flash, and I have two YN560s that I use as slave flashes for studio work. Non-ETTL flashes have their place, for sure, but I wouldn't recommend one as a first flash.
Couple of other notes:
- For most shots, don't bother with a plastic stick-on flash diffuser. It doesn't make the light "softer," it just redirects the light. If you're indoors in a small room with white walls, it'll work great since it will light up the whole room - otherwise, it's just wasting your batteries.
- You can rotate and angle the flash head for "bounce " flash. Experiment with this, learn to use it to your advantage. It can make the difference between a deer-in-the-headlights snapshot, and pleasing side-light or evenly-lit portrait.
- I got distracted and forgot the third thing I was going to say here.