There are two main types of motor: out-runner motor and wheel hub motor.
The out-runner motor uses a belt and pulley system which drives the wheel. Because of this, different sized pulleys can be used to gear the drive system. Popular boards like the Boosted Board Plus use this type of motor system. Hub motors are incorporated into the wheel hubs themselves, which give a smaller overall footprint. Hub motors are often used in smaller electric skateboards.
Speed Controller and VESC
All electric skateboards need an electronic speed controller (ESC) in order to vary the speed of the motor for accelerating or braking. The rise of hobbyists building their own electric skateboards meant demand for a more functional and specific ESC. The VESC (which stands for Vedder Electronic Speed Controller) is a more advanced ESC which allows for features such as better motor and battery protection, regenerative braking, programming options like acceleration and deceleration curves, and other advanced features. Previously these hobbyists had been using RC ESCs which were not as robust. Not every VESC is equal though, as Benjamin Vedder designed the blueprints as an open source project. Companies selling the VESC have modified and improved on these original blueprints