A controller is any device that can be used for controlling music software. These can be split into general (keyboardless) controllers which will often feature drum pads, pots and faders which transmit MIDI information via USB, or DAW controllers, which have specific controls over your DAW package and often have screens that replicate DAW information and settings on the device itself.
Control surfaces come in many size and shapes with smaller units offering basic playback control functionality and in the other extreme the larger units will replicate a complete mixing desk. Having a physical and tactile controller will allow quick and easy control over your software package of choice helping to improve your workflow in the studio.
