In more detail, a regulator in SCUBA terms consists of
a primary (or first) stage regulator, atleast one secondary
(or second) stage regulator, and usually 4 hoses.
The primary stage attaches directly to the high
pressure air tank and feeds high pressure and low
pressure hoses. The high pressure hose usually goes to
your dive computer or gauges, the low pressure hoses
go to your BCD and secondary stage regulator(s).
The low presure hoses contain air at the same pressure
as the water arround you.
The secondary stage regulators take the low pressure
feed from the primary and supply you with air in a controled
manner. (eg only when you breathe in). You usually have
a primary second stage (the one you normaly breathe through),
and a backup second stage, (aka the Octi).