A network switch is a computer networking device that connects network segments.

A standard 10/100 Ethernet switch operates at the data-link layer of the OSI model to create a different collision domain per switch port. For example if you have 4 computers A/B/C/D connected to 4 switch ports, then A and B can transfer data between them as well as C and D at the same time, and they will never interfere with each others' conversations. In the case of a "Hub" then they would all have to share the bandwidth, run in half-duplex and there would be collisions and retransmissions. Using a switch is called micro-segmentation. It allows you to have dedicated bandwidth on point to point connections with every computer and to therefore run in full duplex with no collisions.