What's the Difference Between Collision Domains & Broadcast Domains?

This covers the difference between a collision domain and broadcast domain in terms of computer networking.

WHAT IS A COLLISION DOMAIN?

Collision domains describe the area in a network where datagrams can collide. The number of collision domains depends on equipment. 

The image below shows a 4-port hub and 4-port switch. The hub and connecting devices are all a single collision domain, why? Because a hub forwards datagrams out of all interfaces therefore there's the possiblity of a collision on all interfaces whenever traffic is transmitted.

The switch in the image has 4 collisions domain, why? Switches are more intelligent, they make forwarding desicions based on the MAC address table therefore each each link between an end device and the switch is an isolated collision domain. There's 4 links therefore 4 collision domains.

WHAT IS A BROADCAST DOMAIN?

Broadcast domains describe how far a datagram broadcasted out can reach. If an end device broadcasts a datagram out, it will be forwarded out to the whole subnet. It doesn't pass routers or pass between subnets so a broadcast domain is contained in to it's own subnet. 

(You can forward broadcasts between subnets such as using DHCP Relays to forward a DHCP Request, which is a broadcast, between subnets)

The topology below shows two layer 2 networks separated by a router. The router is the barrier for the broadcasts so each side of the router is a broadcast domain.

VLANs can be used to minimise broadcast domains. If half of your switch is for VLAN 10 and the other half is set as VLAN 20, then broadcasts for VLAN 10 will only be forwarded to half of that switch meaning each VLAN will be it's own broadcast domain.