What is the difference between a sender and a switch?

A sender is a variable resistance device used in conjunction with a gauge or meter. For example: A temperature sender will provide continuous readings to a gauge as the temperature raises and lowers.

A switch is a make/break device used to operate a lamp or buzzer. For example: At a preset temperature a temperature switch will cause a lamp to light or buzzer to sound so the operator can take appropriate corrective actions.

Stewart Warner provides:

Senders - Temperature, Pressure and Fuel

Switches - Temperature, Pressure and Vacuum