Gearmotor : Principles and usefulness compared to a regular motor
In short: A gear is a transmission component, that matches the motor and load speed. A motor delivers nominal power at certain speed (nominal speed) where outputs a nominal torque. Just regulating the speed by lowering it, also lowers the power, so it might be useless to drive a load with 1/10-th of rated motor speed, because you would need 10 times bigger motor than needed. 

For drive dynamics, the transmission is also used to match rotor mass of inertia with load mass of inertia. This is important for high dynamics.

There are plenty of transmissions possible: planetary gear, rack/pinion, spur gear, harmonic gear, belt,...it all matters for the price, transmission loss, type of translation needed.