Wikipedia
A CDJ is a specialized digital music player for DJing. Originally designed to play music from compact discs, many CDJs can play digital music files stored on USB flash drives or SD cards. Unlike consumer CD players, CDJs use high quality digital-to-analog converters. In typical use, at least two CDJs are plugged into a DJ mixer. Many CDJs have jog wheels and pitch faders that allow manipulation of the digital music file similar to a vinyl record on a DJ turntable. Some CDJs have additional features such as loops and beat analysis that are not possible with turntables. Some CDJs can also be used as DJ controllers to control the playback of digital files in DJ software running on a laptop instead of playing the files on the CDJ.
The most popular CDJs are made by Pioneer DJ. Gemini Sound Products, Numark Industries, and Denon also produce cheaper CDJs.
CDJs were the first practical way for DJs to use digital music files before DJ software developed and laptop computers became powerful and cheap enough to run DJ software. Now, DJ controllers together with software and a laptop can provide all the functionality of CDJs plus a DJ mixer much more cheaply. Nevertheless, CDJs remain popular in DJ booths at clubs, where they are provided by venues for DJs to use without transporting their own equipment.