The Motor Shield is based on the L298, which is a dual full-bridge driver designed to drive inductive loads such as relays, solenoids, DC, and stepping motors. It lets you drive two DC motors with your board, controlling the speed and direction of each one independently. You can also measure the motor current consumption of each motor, among other features.

Operating Voltage: 5V to 12V
Motor controller: L298P, drives 2 DC motors or 1 stepper motor
Max current: 2A per channel or 4A max (with external power supply)
Current sensing: 1.65V/A
Free-running stop and brake function
Size: 6.8x5.3x2.3cm (approx)