Well, the actual mount is different. So FD lenses won't fit onto EOS bodies. You can use an adapter, and there are two main types; with lens and without. An adapter without a lens element would allow you to mount the lens, but would take away the ability to focus at infinity. So it would really only be useful for Macro or fairly close shooting. The type with a lens element corrects for the lens offset and allows normal focus range, but your image quality is now subject to the quality of the lens in the adapter....and as you can expect, the cheaper adapters have pretty cheap lenses in them.
There there is the issue of using the lens. Obviously they won't auto focus, but the camera can't stop down the aperture either, so you'd have to do that manually on the lens....and that may require that you use 'stop down metering'.
So, the general consensus that I've always heard, is that it's only really worth it, if you have some really great FD lenses. So if you were just wanting to buy an FD 50mm F1.8 for $25....just to save money...I wouldn't recommend it.
An interesting point, is that when Canon switched from FD to EF lenses and EOS cameras, they did make some FD to EOS adapters that were of good quality, but they only gave/sold them to photographers who had significantly invested in FD lenses...because they were understandably upset over the change in format. You can sometimes find those high quality adapters on E-bay etc, but I think they are fairly expensive.
I have an FD 50mm F1.2 that I'm itching to try on a digital body, so I might order an adapter....if/when I do that, I'll be sure to relate my experience here on the forum.