2. Relays are generally used in electrical control circuits to magnify the contact capacity of micro or small relays to drive larger loads, like you can use the contact point of the relay to connect or disconnect the contactor coil. Generally relays have many open and close contacts, of course, through the appropriate connection relays can also achieve some special functions, such as logical operation.
3, The similarities between the two: all are controlled by the coil of electricity or no electricity to drive the contact opening and closing to disconnect or connect the circuit, which belong to electrical appliances with contacts. The control circuit of the coil is electrically isolated from the electrical circuit of the contact.