Collision detection is the method of detecting the data collision in the network. Suppose there are two computers A and B connected in the network. Computer A and computer B first checks if the medium is free to transmit the data. Suppose medium is free so both computers starts sending the data. After some time data from computer A and B collides. When data collides with each other then data becomes corrupted. The corrupted data reaches computer A and computer B. When both computers receives the corrupted data so they know that their data is collided. Both computers only know if data is collided when they are sending the data. If computers stop sending the data then they not know about collision.
Collision detection is also known as collision sense multiple access/collision detection (CSMA/CD). The collision detection method was used in old computer networking when half duplex is used in the network. In half-duplex, only one computer sends data at a time and only one computer receives data at a time. Nowadays networks use full-duplex in which multiple computers/devices send and receive data at a time in the network.