What is the difference between collision domain and broadcast domain?

A collision occurs when two devices send a packet at the same time on the shared network segment, that can be happen in bus toplogy. and Collisions are often in a hub environment, because each port on a hub is in the same collision domain. 

Broadcast domain is the logical division of computer network, in which all nodes can reach each other at the broadcast at the data link layer.

Collision domain is a part of network where packet collision can occur. A collision occurs when two devices send a packet at the same time on the shared network segment.