From LQWiki

In networking, a collision happens when two computers try to transmit a packet at the same time. In an ethernet based network, network devices check to see if another device is transmitting before attempting to transmit themselves, but there is a brief period where two computers can transmit at almost exactly the same time.

Network switches that support fragment-free filtering buffer packets for that time period in order to check if they are collision fragments.

When a collision happens, both computers "back off" for a random amount of time before attempting to retransmit. In theory, both computers could block each other indefinitely if they both happen to select the same back-off period, but the odds are against it.

Collisions significantly slow down traffic in a network segment, and a large amount of collisions is a sign of an over-worked network segment. Collisions can be reduced by using network switches or bridges which reduce the size of collision domains. Routers also reduce the size of collision domains, but they also increase latency, so they are not recommended for use in most LANs.

Certain types of networks, such as token ring, do not experience collisions.