What is a collision domain and a broadcast domain?

a collision domain represents a port in your switch; a broadcast domain is a network separated by a router.

Collisions are localized to the network segment they occur on. Thus, the broadcast domain is the entire inter-connected layer two network, and the segments connected to each switch/bridge port are each acollision domain. Not all network systems or media feature broadcast/collision domains. For example, PPP links.

What we know Network OSI model has 7 layer . Top of layer called Application Layer & bottom layer called is phycical layer . In Ethernet if two or more device connected physically like pc to pc without any switch or using a device called HUB. In this sitution only one can send at a time rest of them keep listening , if two or more device try to send the electrical signal at same time then its  collision happen in electrical signal level & its electronicaly detected by everyone so everybody should stop sending any packet for random time interval and then again start sending one by one ...so you understand its is always half duplex connection. 2nd Bottom layer called datalink layer & its has addressing thing called HW ADDRESS or mac address .what it only know which mac address is attached with which switch port ,so switch can forward datalink traffic from source mac to dest mac . This layed dose not understand any IP or avove things.

Hope its useful for you