A starter relay is a high current switch that's used in the starting circuit of some cars. In it is a set of windings and metal discs which move when power is provided to the relay.

The idea behind a starter relay is to use a low current switch (your ignition switch) to power a high current device (the starter motor). When you turn your key to 'start', the ignition switch provides 12 volts to the starter relay. When the relay recieves power, it acts as a magnet, drawing closed a high current circuit which provides power to the starter motor.

Lots of autmobiles that have starters without the solenoid bolted to the starter motor itself, use starter relays to switch the power. Thoese starters which have the solenoid bolted onto the starter have their own relay built inside the solenoid.

In either case, you should disconnect the battery when working on any part of the main electrical system (starter included), as this system is high current and can eaisly maim or kill you. Subsystems such as the interior lights, are switched to a much lower amperage and are not as risky. Never the less, it only takes 1 Ampere to injure you.