Pretty much like a full-sized aircraft! Except the pilot stays on the ground and operates the control surfaces remotely. Inside the model, there is a receiver, which decodes the signal from the pilot's transmitter, and servos which are connected to the control surfaces.
The servos contain electric motors which move in proportion to the position of the sticks on the transmitter. The transmitter has 2 sticks, which can have 2 different functions mixed together on each stick.
In the typical configuration, the right hand stick controls the ailerons (which control banking) and the elevators (which control the aircraft's pitch: nose up or down) - this is exactly the same as the main hand-held controls in a full-size aircraft.
The left stick would then control rudder and throttle. There may be more or fewer functions in use depending on the model type: a beginners glider may only use rudder and elevator, while a scale replica of a full-size aircraft may add flaps and retractable undercarriage to the above 4.