The standard audio connector for DJ equipment is known as an RCA or phono (Cinch in mainland Europe). All DJ CD players will have at least one pair of RCA outputs per deck, and o monitor and digital outputs may also be provided, the latter either on RCA or TOSLink optical. (Although the connector is the same, digital outputs on RCAs require a special co-ax type cable to be used.) Digital mixers are becoming more common these days and provide the benefit of perfect audio transmission between player and mixer.
In addition to audio connections you may also find connections for Fader Start (to use with compatible mixers) and Relay, which makes one CD player play as soon as the other reaches the end of a track. Two-piece DJ CD players will also have a connection for the remote control section. Look for a good solid connection, preferably with a locking connector so that the cable cannot accidentally be pulled out of the socket.