This is a bog standard electronic speed controller (ESC) for brushless motors - now with a T-Plug on the power lead, which can be cut off if required. These are ideal for our outrunner motors.
If you've never used one of these before - ESCs do something pretty simply (drive a motor), but the manner in which they do this is pretty complicated. Because the motors being driven can demand large amounts of current, and have high torque, the ESC has a number of safety features to provide 'soft starts' which reduce the risk of damage to your project or model. There are also other protection features, like over-heat and throttle signal loss. The ESC can also provide braking to the motor and also offers different types of throttle responses to fix fixed wing or rotary platforms.