They're pretty different, but the camera mount is the same, so cameras designed for FL lenses can take FD lenses, and vice versa (except for a few odd ones). The main difference you'll probably notice is that you can't do full aperture metering with FL lenses. You have to meter with the lens stopped down, which makes it next to impossible to focus sometimes. FL lenses use the breech lock mount, which means you turn a ring on the lens to lock it onto the camera. The older FD lenses use the same mount. New FD lenses use the semi-bayonet style mount, which means you line the lens up on the body, and twist it on. It locks automatically. I personally prefer the bayonet style mount. Here's a really good thread that goes into more detail: http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004sgt
There are lots of third party FD lenses available. Of particular note is the Vivitar Series 1 line. Some of them are absolutely spectacular (and you'll find that the zooms are often better then the equivalent Canons from the time - non-L that is!). The 90mm f/2.5 macro is VERY sharp. The 70-210 zooms are beautiful. If you'd like, I can forward you an e-mail about the zooms.