EF lenses are originally designed for 35mm film cameras. Their image circle (image projected into the camera) is big enough to cover 35mm film or a full frame digital sensor (as in the 5D or 1Ds).
Cameras like yours, have a smaller sensor, which is OK but it only sees the middle section of the image circle...'cropping' the edges off. This makes it hard to get a wide angle view.
For this reason, Canon made EF-S lenses. They are designed only for cameras with sensors smaller than full frame. Their image circle isn't as big and their rear element sticks into the camera a bit farther than EF lenses. Basically, they are a cheaper & smaller way to get wider views on crop cameras.
So EF lenses can be used on any EOS camera, film or digital. EF-S lenses must be used on 'crop' bodies only.
Build quality isn't part of it. Sure, your EF-S 18-55mm lens feels like a toy...but there are plenty of EF lenses that are just as bad. There are EF-S lenses that are much, much better (well most of them are better than the 18-55mm).
