What is a collision domain ?

A collision domain represents the end devices that are interconnected via a hub or a series of hubs. A collision domain is also referred to as a network segment. Hubs and repeaters therefore have the effect of increasing the size of the collision domain.

Switches allow the segmentation of the LAN into separate collision domains, which means that one port on the switch is one collision domain; the number of ports on the switch gives the number of collision domains, but if you have in the network L1, L2 and L3 devices you have to analyze the network and compute the number of collision domains.

I have a table that you can see (the source table is from here: https://learningnetwork.cisco.com/thread/46273?start=0&tstart=0):

Device COLLISION DOMAIN BROADCAST DOMAIN
———————————————————————————————————————
Hub/Repeater Doesn’t Split Doesn’t Split
Switch/Bridge Does Split Doesn’t Split (By default)
Can be done through VLAN
Router Does Split Does Split

To have a better understanding watch Dan’s explication from danscourses.com:

If the video does not work, click here: http://www.youtube.com/embed/_c1gqcr6Lcs