TC -
All the acronyms EF IS USM F and so on can be intimidating to beginners. I am going to try to give you some basic on the lense.
EF-S vs EF lense. Crop camera vs Full Frame Sensors:
EF-S lense can only work on Canon DSLR that has cropped sensors. The crop sensors are only available in digital camera (usually the lower end lines). The high end cameras (professional lines) have full frame sensors. The EF-S lense will ONLY work in cropped sensor cameras (Xt, XTi, XSi, 30D and 40D) but not in the full frame camera (5D, 1D, 1Ds). EF lense will work on ALL Canon DSLR (crop and full frame) and Canon film camera.
Another thing you need to know about crop vs FF is how it affects your picture. When you buy a lense, it has a zoom range (lets say 18-55mm). If you have used film SLR, you know what an 18-55mm lense can do. If you have a FF camera, the focal length remains the same. However, crop cameras usually have a multiplier . In case of the Canon, the multiplier is 1.6. So your 18-55mm on a crop camera is the equivalent to a 28.8-88mm (multiple 18-55mm by 1.6).
Image Stabilization
The IS stands for image stabilization. Basically - when you are holding a camera, there is some shaking in your hand. The image stablizes uses mechanism to compensate for your shaking. This is important when you are shooting images at a large focal length (100mm or higher). Also - a general rule of thumb is that if shutter speed is less than 1/60 of a second, you should use a tripod because you can hold the camera still. With IS - you probably can hold the camera down to 1/30 or even 1/15 of a second.
http://web.canon.jp/imaging/lens/index.html
Ultrasonic Motor - USM
USM - this is canons Ultra Sonic Motor. Ultrasonic Motor are quieter and faster than the non-USM.
F Stop
Aperture (f stop). If you look at the Canon EF 70-300mm f4-5.6, it means that the largest aperture (smaller F stop) is from 4 to 5.6. At 70mm, it's probably F4. At 300mm, it's F5.6 The smaller F stop is better because you are more likely to be able to use it in low light. For example, my wife was taking some pictures of elephant seals at dusk with her lense at 200mm F5.6. Even with optical stabilization, the pictured turned out blurry. A friend of ours shot with a 200mm F2, the picture turned out sharp. But the difference is that his lense cost over a $1K.
Unless you are extremely picky, the quality of any of those lense would be good. Can I ask what type of pictures are you thinking about taking? Will it be mostly landscape, portraits, macros? What will the lighting condition be like?
BTW - since my wife didn't know what type of photopgrahy she wanted to get into, I got her a "general purpose" lense. It's not great at any one thing, but it's good at a lot of things. It's a Sigma 18-200mm f3.5-6.3 DC OS (the OS means that it has optical stabilization and the DC means that it's equivalent to the EF-S). The lense is about $450.
Anyway - give us more info and maybe we can give you more advice.
Minh