Power input on l293d motor driver
One is logic power (pin 16 (20 for SMD) at top right,should be 5v or whatever your logic level is), the other  is motor power (pin 8 (10 for SMD), bottom left, can be from 4-36v and should be whatever your
motor needs).

This means you can run your motors at different voltage, and also isolates 
power spikes from the motor supply from the logic.

Have a look at the bipolar stepper motor example circuit here: http://www.tigoe.net/pcomp/code/circuits/motors/stepper-motors  (The circuit says SN754410NE, that device is just a higher power version of the L293).