I like the simplicity of this answer. The question is fully answered as it is asked without going overboard with explanations. GA.

Okay, but don't use the coil with AC. Makes too much noise.

For a better explanation, relays are great for changing voltages for control circuits. By this I mean you can design a circuit using AC and energizing an AC relay but sending DC through the contacts to energize a DC motor or other component. Likewise you can have a DC circuit to energize a DC relay and send AC through the contacts to activate an AC component.

Relays and contactors have contact Voltage and current ratings. If you are lucky it will be marked on the device. Otherwise you will have to look for the specifications for the Manufacturers part number on google.