Stands for
Carrier Sense Multiple Access with Collision Detection. It is the primary
traffic regulation protocol in
Ethernet networks. Compare with
token ring's contention avoidance mechanism.
It works just like it says. Before a station transmits a frame, it will sense for carrier (a voltage on the wire indicating another station is transmitting.) If it senses carrier, it will hold the frame until the line is clear. Multiple access means many stations may share the same physical media or collision domain.
Collision detection happens when two stations try and transmit relatively simultaneously. Mind you, this is within the maximum time it would take for a signal to propagate across the entire collision domain. This is usually on the order of milliseconds, and is why there are distance and repeater restrictions in Ethernet design--both cause this time to increase and increase the likelihood of collisions. Network collisions are not bad, necessarily.