Carrier sense multiple access, MAC layer
In collistion detection, a node may start transmitting at any time, and backs off if it detects that it caused a collision. This forces the node that was already transmitting to back off, too. Its advantage is simplicity; its disadvantage is that it introduces big inefficiences: a packet makes it through the network only if no other node decides to transmit at the same time.

In collision avoidance, a node first looks at the medium. If it detects a signal, then it backs off and waits before attempting to transmit again. This increases the transmitter's complexity a bit, but the network efficiency is increased. A collision only happens when two nodes decide to transmit at (almost) the same time.

You can find the details and how to calculate the specifics in any textbook on networking.