DJ MIDI (musical instrument digital interface) controllers are designed to enable DJs to interact with the music stored on their computers. What kind of DJ you are will determine what hardware you prefer. Most mobile DJs want a simple layout with minimal effect and scratching capabilities. As for performance DJs they will want an interface that feels good and lets them interact with their computer seamlessly. American audio has a great line of mobile DJ equipment, and Pioneer have some very popular pieces that are designed around Serato.