Circuit breaker,Contactor, Relay & switch

I'm going to assume that you're just curious and not wanting someone to do your homework for you.

A circuit breaker senses current, and breaks contact by bimetal, thermally actuated contacts, electromagnetically actuated contacts, or electronic means, to illustrate a few of the most common (although electronic means is not exactly common).

Relays are, in the most common form, an electric solenoid (electromagnet) which pulls on an armature to which a contact (one or more) is physically connected, and upon enegrgizing the electromagnetic coil, the contacts close, and upon de-energization, the contacts open. Around 45-50 years ago, solid-state relays began to appear on the market, and they perform the same functions as simple electromechanical relays do with semiconductor devices.

Switch is nothing more than a hand-operated set of contacts (one or more). Open in one position and closed in the other.

Contactor is a relay.

Try Wikipedia

http://en.wikipedia.org/wiki/Wikipedia