Switch
In computer networking, a switch connects multiple network devices together to form a LAN. A switch works by filtering and forwarding packets between LAN segments, transparently bridging them.
A switch is essentially a more efficient, sophisticated hub.
Not a router
A switch is often confused with a router, but the two serve very different purposes. A router determines the best way to route packets across networks, based on certain network conditions.
Analogy
A good analogy for switches and routers is to think of a network of computers as a neighborhood. A switch is the street which connects the houses, and routers are intersections which connect the neighborhoods and contain important information about them, like street names.
