Switch contacts are arranged on the 'arm' so that when the arm moves it causes the switches to either open , or close . On some relays, as one switch opens another closes. A relay may have two 'arms' that move at the same time. When a relay 'switches', contacts inside the body of the relay close, just like a light switch. The size of the contacts determines how much current the relay can handle.
Most small relays can safely pass several amps without the contacts overheating.
In general you should only attempt to control small motors and lamps with a relay. NOTE about 240 Volts - Relays do exist for use with 240 volts. Only a licensed electrician is permitted to work with 240 volt equipment. The current from the printer port of a computer is not sufficient to switch a relay. Either a transistor circuit, or a ' driver ' IC must be used to avoid damaging the computer itself.