About DJ Controllers:
With DJ technology moving towards the digital world, enabling smaller, more mobile systems, the DJ controller is rapidly becoming an essential piece of gear. Designed to emulate the traditional mixer/turntable/CD deck setup, modern controllers are aimed at integrating smoothly with powerful software like Serato or Traktor while maintaining the flow and feel of a traditional setup.
Digital DJ controllers are frequently aimed at a specific software package (e.g. the Pioneer DDJ-T1 and S1 are set up for Traktor and Serato, respectively), most are reprogrammable to work with virtually any software package, as the controls are all mapped to MIDI messages. While it's always good to start with a controller designed for the software you prefer, it's good to know that you can change if the need arises.
Controllers almost all have built-in jog wheels to emulate the experience of vinyl or CD deck control, with a variety of sizes and schemes to match almost every DJ's needs. But even if you're a vinyl hardcore, you can still get the advantages of digital systems by using control vinyl on actual turntables that feeds timecode to the software to give you turntablist action on a digital system.
From systems designed to work with a mobile phone or tablet to massive four- (or more) deck virtual systems, DJ controllers hit such a wide range that the best way to approach them is find the hardware/software combination that you're most comfortable with, whether you're just starting or moving over from a more traditional analog system. Having that giant virtual crate of music stacked up in the computer and more control than ever before can make your sets just that much more exciting.