fixed gear frames convertible?

[DebW]

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.