CSMA/CD stands for Carrier-Sense Multiple-Access with Collision Detection and is used for controlling access to the network media. CSMA/CD is used to detect collisions on the network which happen when two stations attempt to transmit at the same time.
If two stations attempt to transmit at the same time, all stations detect it. Both stations that were involved in the collision choose a random time interval to wait before attempting to transmit their messages again. If a collision happens again, the time intervals are increased. The IEEE 802.3 standard and ISO 8802.3 standard describes CSMA/CD.