fixed gear frames convertible?

ok.. newb question..

fixed gear frames... does that mean they can only be used for fixed gear? you can 't add a cluster and make it into a "normal" bike?

and what is the diff between single vs fixed?

You need to know the spacing between the inside faces of the rear dropout. If the frame was designed as a track frame, it's probably got 110 mm spacing and can't take multiple gears. If it was originally a derailleur bike with 120 mm or larger spacing that was converted to single-speed, then you can easily go back to 5 speed (120 mm), 6 or 7 speed (126 mm), or 8, 9, or 10 speed (130 mm). Also check the shape of the dropouts - vertical, horizontal, or rear-facing. See

http://www.sheldonbrown.com/gloss_dr-z.html#dropout
and
http://www.sheldonbrown.com/gloss_sp-ss.html#spacing

Single-speed and fixed both have a single cog in place of a freewheel or cassette, but the single-speed allows the cog to freewheel and the fixed gear doesn't (ie. you must pedal at all times).

edit: You would probably need to replace the rear hub to convert from single speed to multi-gear. If the dropout spacing is 120 or 126, you'll have to find an old hub with that spacing that takes a freewheel. And check also if the frame is equipped with a derailleur hanger.

A frame orginally built for fixed gear probably does not have either brake mounts, derailleur hangers or cable stops. Some fixies do have replaceable drop outs so that could be overcome. You could use clamp-on cable stops. If there is no drilling or mount for brakes, something would need to be brazed or welded onto the frame. If you go to this effort for the brakes, the same person could probably replace the dropouts (and then repaint everything). This can get pretty expensive to go to all this effort.

cool.. thanks guys. great info and thanks for the links, Deb..