A switch uses layer two of the OSI model, so the switch uses MAC addresses to send the packet to the correct device. Rather than sending it to all ports a switch only sends the packet out one port, if it has the MAC address in its MAC address table. If not the switch will send the packet on all ports except for the port in which the packet was received on. Switches provide separate collision domains on each port. This provides dedicated bandwidth to that device. This also allows simultaneous conversations between devices on different ports. Each port can be operated at full-duplex so the device can send and receive information at the same time.
A broadcast domain is like a collision domain, the definition of a broadcast domain is a set of devices that if one device sends a broadcast frame all other devices will receive that frame in the same broadcast domain. So if devices are in the same IP network they will be able to receive a broadcast message. Having a smaller broadcast domain can improve network performance and improve against security attacks. The more PCs and network devices connected to a single broadcast domain, the more broadcast messages you will have. Remember a broadcast message goes to every PC and network device. An example is when the router gets a packet that is destined to a host (192.168.1.124) on its Ethernet interface (192.168.1.0/24 network) the router will send an ARP request saying who is 192.168.1.124? That packet will go to every PC on the network, each PC has to look at the packet and then discard it if it is not 192.168.1.124. But only be processed by the PC that is 192.168.1.124. So a broadcast message can be just like a collision domain and affect network performance. The only devices that can block or not send broadcast messages are routers because they separate networks. Each interface on a router is a different network.
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