routers breakup broadcast domains, switches breakup collision domains.
say you have two networks joined by one router netA & netB. All devices in NetA are in the same broadcast domain, all devices in NetB are in their own broadcast domain.
Directly connected to each port on the router is a switch. Any devices connected to a specified switchport are in their own collision domain.
A broadcast domain is simply an area in which broadcasts can be sent.
A group of Ethernet or Fast Ethernet devices in a CSMA/CD LAN that are connected by repeaters and compete for access on the network. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions.
A collision domain is sometimes referred to as an Ethernet segment.