Starters are specialized switches which turn on and off heavy loads such as electric motors. The principal of operation is based on using an electro-magnetic coil which draws a small amount of power to pull in a heavy set of contacts (switches) which turn on an electric motor or other heavy loads such as heaters or a long string of lights in a building. This enables a small push-button switch or other electronic control to turn on and off the load.
The difference between a starter and contactor is that the contactor is the switch mechanism only with an electro-magnetic coil. A starter has additional components such as overload protection and possibly switches and an enclosure too.