Switch
A switch is a type of Ethernet hub which, rather than repeating received data on all ports, uses the address associated with the data to determine which port to send it out. This reduces the data on the other ports, and improves traffic flow.
These days, switches are often used in place of hubs and very often a high-end managed hub is a similar price to a managed switch.
Some switches are capable of applying rules to the determination process and this can be used to create virtual networks (VLANs) for security or traffic management purposes
Layer 3 switches are able to set rules which apply to the protocol part of the data, so are able to discriminate between (usually) IP subnet destinations. This can be used to split the network into separate IP LANs. Some Layer 3 switches also operate as partial or full routers, allowing access from one subnet to another. (Layer 3 refers to the protocol information level of the ISO standard - normal switches are known as Layer 2 and use the address information.)
Using switches does introduce some network monitoring complexity. Switches break the network up into separate segments, and devices on one segment will not see the data on another.
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See also: hub