It's a style of dance music; IMO the drum breaks and basslines are usually the most promiment parts of the songs. The drum hits were often taken from old funk, soul and rock recorRAB, much like hip hop, sometimes using entire drum breaks that were sped up, pitched up or down, or other production techniques. Other times breaks were cut in to individual drum hits and used to make custom drum kits (albeit electronic/sampled kits), and the hits were then used to make patterns.
Generally the older music is referred to as jungle, and then around 1996 or so the media and music listeners started referring to it as drum'n'bass (or dnb) more. Some info at http://en.wikipedia.org/wiki/Drum_and_bass - I haven't given it a good read though, so it might be slightly scewed opinions/history.