From Chapter 7 of the current FAA Instrument Flying Handbook:

INS Components
INS is considered a stand-alone navigation system, especially when more than
one independent unit is onboard. The airborne equipment consists of an
accelerometer to measure acceleration—which, when integrated with time,
gives velocity—and gyros to measure direction.
Later versions of the INS, called IRS (inertial reference systems) utilize
laser gyros and more powerful computers; therefore, the accelerometer
mountings no longer need to be kept level and aligned with true north. The
computer system can handle the added workload of dealing with the
computations necessary to correct for gravitational and directional errors.
Consequently, these newer systems are sometimes called strapdown systems, as
the accelerometers and gyros are strapped down to the airframe, rather than
being mounted on a structure that stays fixed with respect to the horizon
and true north.
INS Errors
The principal error associated with INS is degradation of position with
time. INS computes position by starting with an accurate position input
which is changed continuously as accelerometers and gyros provide speed and
direction inputs. Both the accelerometers and the gyros are subject to very
small errors; as time passes, those errors likely will accumulate.
While the best INS/IRS display errors of 0.1 to 0.4 NM after flights across
the North Atlantic of 4 to 6 hours, smaller and less expensive systems are
being built that show errors of 1 to 2 NM per hour. This accuracy is more
than sufficient for a navigation system that can be combined with and
updated by GPS. The synergy of a navigation system consisting of an INS/IRS
unit in combination with a GPS resolves the errors and weaknesses of both
systems. The GPS is accurate all the time it is working but may be subject
to short and periodic outages. The INS is made more accurate because it is
continually updated and will continue to function with good accuracy if the
GPS has moments of lost signal.