The following information is courtesy of Randy Sargent (rsargent@media.mit.edu).
The SGS/Thompson L293 series of chips are power H-bridge motor drivers. The L293B and D chips are both in 16-pin dip packages, and both have two h-bridge drivers. An H bridge is typically capable of running one DC motor bidirectionally (forward, backwards, off), or two separate motors unidirectionally (forward, off). Thus a L293 chip can run two motors bidirectionally, or 4 unidirectionally.
L293 chips take logic-level inputs to direct the H-bridges, and have a separate pin for the motor supply (which is often higher than the standard 5V logic supply).
The L293B and D chips have identical pinouts, but have two differences. The first is that the L293D can run motors up to 600 ma, while the L293B can only run all the way up to 1 amp per H-bridge. The other, and perhaps more important difference, is that the L293D has protection diodes which shunt the current spikes produced by inductive loads (such as DC motors) when they are turned on or off (such as during the armature contact switching or pulse-width modulation). The L293B has no such diodes, so the user must usually add his own. Use the L293D for your projects.
Read the L293 Data Sheet.