Switch
A switch, in its most general meaning, is a means of putting a break in a conductor. Most switches are mechanical; one of a pair of contacts is moved so that it is no longer touching the other. (Which can lead to an entertaining display when the circuit in which the switch is emplaced is carrying a very high voltage; if the switch is not in a vacuum, an arc can be created between the two contacts, until they are far enough apart.)
Switches exist which contain multiple pairs of contacts, and the means (usually mechanical) which separates a pair simultaneously separates all the pairs. The other complication which exists is than instead of a pair of contacts, there can be multiple contacts, and the means moves one contact (the 'input') to select one (or none) of the multiple other contacts (the 'outputs'). (Obviously, this can be run in reverse, so that a single output may be connected to one of a number of inputs.)
Both of these can be provided in a single switch; there is terminology to crisply describe the exact configuration of any such. A 'single-pole, single-throw' (usually abbreviated to 'SPST') is the simplest of all; a switch which contains two sets of contacts, each with two potential inputs, is termed a ' double-pole, double-throw' (abbreviated as 'DPDT').
There are also solid state switches, but they are rare.
