Motor shields in arduino
Servos require power and PWM. Servo shields come in two main forms:


Simple distribution
Active control


The first kind just provides convenient 3-pin headers to the Ardino's PWM pins, and sometimes a dedicated power supply for powering the servos. They're simple and cheap, but can only provide as many servo ports as the Arduino has PWM pins.

The second kind, more expensive, has a PWM chip on it to provide many more PWM pins than the Arduino can natively provide. Again there is sometimes a power supply as well, along with the convenient 3-pin headers. You usually interface with this kind using either I2C or SPI.  These are the ones people usually mean when they talk about a servo shield - a shield designed for powering and controlling many more servos than an Arduino by itself can manage.

Servo Motor Shields should not be confused with Motor Shields, which provide a number of H-Bridge drivers for powering and controlling (both the speed and direction) of a brushed DC motor.