Breech-lock (or you might sometimes see them called barrel-lock) lenses have a ring that you tighten to fix the lens on the camera, the newer lenses are the ones you fit into the bayonet and turn the lens to lock in place. For example, this is a picture of my 100mm f/4 macro with its extension tube.
The extension tube has a silver ring at the bottom - this is the lock. You put the tube on the camera and then rotate the ring to lock it in place. With the lens, you line up the red dot with the red mark on the camera and then rotate the lens to lock it into place. That's the newer type of FD mount (introduced c.1980), hence
new FD or
FDn.
I'm a little surprised that Matt's recommended you look for B/L lenses because I was never as impressed with the results as with the newer lenses. I ran some side-by-side comparisons between my FDn 50mm f/1.4 and a 50mm f/1.4 SSC* B/L years ago and if it's any indication, I only own the FDn version now.*
Incidentally, if you decide to buy a non-Canon FD lens, chances are that it'll have a breech-lock as it seems third-party manufacturers stuck with the older mount, probably to avoid the costs of updating their manufacturing. There's no difference in operation once the lens is on the camera, it's purely how they mount. FDn lenses tend to be a little shorter than their B/L counterparts because they don't have to incorporate the ring and I find them easier to fit and remove when out and about.
* SSC is a designation you'll see on some B/L FD lenses and stands for Super Spectra Coating, i.e. the lens is multi-coated. The FDn versions have this coating as standard.
** If you want to spend a bit more money, the FDn 50mm f/1.4 is an absolute belter and is described on mir.com thus:
The 50mm f/1.4 lens was used for optical measurements at various public institutions and is also the standard which determines color balance for the rest of the nearly 60 lenses in the FD series. I absolutely love mine
