The motor shield can drive up to 4 DC motors bi-directionally. That means they can be driven forwards and backwards. The speed can also be varied at 0.5% increments using the high-quality built in PWM. This means the speed is very smooth and won’t vary!. Note that the H-bridge chip is not meant for driving loads over 0.6A or that peak over 1.2A so this is for small motors.
 
2 interfaces for 5V Servo connected to the Arduino's high resolution timer
Can control 4 DC motors or 2 stepper motors or 2 servos
Up to 4 bi-directional DC motors with individual speed control
4 H-Bridges: per H-Bridge 0.6A (1.2A peak) 4.5V to 36V DC
With pull down resistors to switch off the motors during start-up