Arduino compatible relay shield???? What for?

 Is the "Arduino compatible relay" for control of an AC motor using the arduino board?
what are relay sheilds used in when it comes to every day life?

Thanks for any help,

LOTS of different relay shields.

Relays are really good for isolating power from signals.

Relays need to be sized for the load they will drive. Motor loads need at least 100% overrating.