A record (or set of records) which, rather than containing a full track, consists of a collection of beats, sound effects, vocal samples, etc. These records are designed to aid DJs in building a set and in maintaining its musical coherence. Often, hip-hop DJs will use records of this kind as the raw material for scratching and beat-juggling (the term "battle record" is often used in this context-thanks funky49). DJ tools designed for house or techno DJs will often be pressed with locked grooves: when the end of a sample is reached, the needle skips back to the beginning, theoretically without interruption.
The term "DJ Tool" is also used, somewhat disparagingly, to refer to a track which doesn't really do anything, musically, but is easy to mix. Tracks of this kind are useful for bridging the gap between two other songs or for extending a set, but don't really add anything of their own.