In terms of current popular technologies, any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain.
Further, any computer connected to the same set of interconnected switches or repeaters is a member of the same broadcast domain.
The distinction between broadcast and collision domains comes about because simple Ethernet and similar systems use a shared medium for communication.
In simple Ethernet (without switches or bridges), data frames are transmitted to all other nodes on a network.
With a sufficiently sophisticated switch, it is possible to create a network in which a broadcast domain is strictly controlled.
The result is a network based on a nominally shared transmission system; like Ethernet, but in which client nodes cannot communicate with each other, only with the provider.
Allowing direct data link layer communication between client nodes exposes the network to various security attacks, such as ARP spoofing.