An ethernet switch is a device used to build a network connection between the attached computers (allows computers to talk to each other). It differs from an ethernet hub: While a hub will send incoming data packets to all ports, a switch understands the packets' addressing scheme and will send any data packet only to its destination port, thus limiting the number of collisions (data sent at the same time).
A switch is used in a wired network to connect to other devices using Ethernet cables. The switch allows each connected device to talk to the others. Wireless-only networks do not use switches because devices such as wireless routers and adapters communicate directly with one another.
Although you can use the ports on the back of a router or modem to connect a few Ethernet devices together, switches offer a number of advantages:
Switches allow you to connect dozens of devices. Switches keep traffic between two devices from getting in the way of your other devices on the same network. Switches allow you to control who has access to various parts of the network. Switches allow you to monitor usage. Switches allow communication (within your network) that's even faster than the Internet. High-end switches can be tailored to your network needs with pluggable modules.