Often referred to by their more historic name MIDI Controllers, Keyboard Controllers are also sometimes referred to as USB Controllers. Nowadays not all controllers have a MIDI jack for connecting to other MIDI devices, but most still do.
In addition to their keys, Keyboard Controllers may come with extensive banks of knobs, faders, switches, drum pads, and/or buttons that can be used to manipulate the sounds and other aspects of software. Many can also be used to operate the functions of recording, looping and editing software such as Ableton, FL Studio, Reason, Pro Tools, Cubase and Logic. Sound libraries and simplified recording software are often included with the keyboard.
Different brands of Keyboard Controllers may be known for their different strengths or weaknesses. For example, Alesis products are know for being inexpensive but with very little control or expressiveness. Novation Keyboard Controllers are known for their Auto-Map feature which automatically assigns the onboard buttons, faders, dials, switches and pads to many different devices and software (often with digital labeling on the controller), saving time and frustration otherwise spent assigning each individual knob etc of the controller. For this reason, Novation Keyboard Controllers also tend to have more of these knobs and switches etc than other brands.
For the young player, keyboard controllers teach skills related to programming and interfacing. The keyboard controller's ability to connect with free and inexpensive software makes it easy for beginner players to jump right in to the world of making music. Before long even the novice can be creating impressive soundscapes with a simple keyboard controller and computer software. For the pro, better keyboard controllers give them the ability to control recording and editing software as well as unprecedented creatvity to manipulate and bend sounds in truly unique ways.