The powershot is a consumer camera, whilst the rebel XT is a professional dSLR.
The difference in definition between the two is that basically with the dSLR you can change lenses to your requirements.
In the real world, having a dSLR means a hell of a lot more - you will be manually able to adjust everything to a greater degree than a consumer camera nd it will be ALOT faster at everything it does.
If you are taking a photography course I would recommend getting two lenses to start with... maybe ask your uncle to buy one bundled with the camera and buy one yourself?
You will probably need a lense with soething around 15-60mm and one say 70-200/300mm, that will keep you going until you've mastered the XT
