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.