EF means it's a full frame lens, the EFS ones are only for crop sensors.
If you have a Canon 5D (Mk1/2/3) or 1D, then you can only use EF lenses, any other Canon DSLR you can use with EF or EFS.
On Nikon cameras, DX is the lenses for crop sensors and FX is the full frame lenses. But Nikon have a neat thing on their full frame cameras, where if you put a DX lens on a full frame camera, the camera will use only the portion of the sensor that is actually being exposed to light.
The thing to look for with Canon lenses is whether they they are an "IS" model or not. With some lenses, they produce a cheaper "non-IS" version as well as the "IS" version.
"IS" stands for Image Stabilisation.
The non-IS lens may well be cheaper but will the lack of image stabilisation annoy you.
(On my Sony DSLR the image stabilisation is built into the camera, so I can use any lens on it without having to worry about the lens having stabilisation or not.)
