MIDI controllers are hardware devices that create and send MIDI data to other MIDI enabled equipment. For computer-based DJs, it can mean anything from triggering samples or fading tracks, EQ-ing or playing tracks at the press of a key or button, using your MIDI controller as the link between you and your favourite software such as Ableton. An MIDI controller is, essentially, a piece of equipment that can control a performance whilst triggering audio events: they can be Pad Controllers, Keyboard Controllers or Control Surfaces, and usually used in conjunction with other DJ Equipment.
Usually MIDI controllers will have sliders and knob controls and velocity sensitive pads that can operate and control sounds produced by modules, drum machines, synthesizers and samples stored in music software. Most MIDI controllers are USB equipped and can directly plug into your computer. As well as their obvious advantages to the performing and recording musician, they are particularly useful for DJ's who use them to operate sounds and trigger samples and loops.