This is a very useful chip. It can actually control two motors independently. We are just using half the chip in this lesson, most of the pins on the right hand side of the chip are for controlling a second motor, but with the Raspberry Pi, we only have one PWM output.
The L293D has two +V pins (8 and 16). The pin '+Vmotor (8) provides the power for the motors, and +V (16) for the chip's logic. We have connected pin 16 to the 5V pin of the Pi and pin 8 to a battery pack. Datasheet