Beat layering is a technique used by drum and bass artists to create a beat from multiple more simple beats. By taking a few samples (usually using turntables) of beats, a good DJ can mix beats in a way that will create a totally new beat, without beat clash, or mistimming it.
This technique is one of the harder mixing skills to accomplish because the DJ must perfectly time the introduction of both beats, as well as memorize each beat just so the DJ knows how the beats will interact with eachother.
Some artists who have mastered this skill are...
Dom and Roland
Goldie
Roni Size
Ram Trilogy
Future Cut
Facs
B_Key
E-Sassin