With a DJ controller you control the DJ software on your computer or laptop. Most controllers can control at least two decks (channels), for more demanding mixes there are devices that can mix 4 decks. The controller is usually connected via USB and recognized as an external sound card. The analog outputs are used to control the sound system and the headphone output is used to listen to the mix. Many controllers also have a microphone input to record announcements. While the cheaper controllers only work with a computer and the appropriate software, some mixers also offer standalone operation and can be used "classically" with external CD players or turntables.
Here you can buy DJ controllers of the well-known brands like Pioneer DJ, Denon DJ, Native, Numark and many more.