Lets take the example of a classroom. When a teacher delivers a lecture, who all can listen to him Its only the students in that class. Other classes will not be able to hear what teacher just said in this room. So that claroom where teacher is lecturing is 1 broadcast domain. Similarly, each classroom is a separate broadcast domain.
technically, if my pc sends a packet than how many pc's can listen to this determines the size of the broadcast domain. Basically this broadcast domain is an entire classfull subnet.example: 10.0.0.0 255.0.0.0 is 1 big broadcast domain.
This can further be breaken up in to smaller broadcast domains by subnetting & vlans.
