In general terms an INS is a self-contained navigation system which requires no external source input whereas a GPS does (i.e. that of satellites).
So far, so good.
I would add...GPS is continually updated for accuracy, via data received from those satellites, whereas, raw INS data is not updated, and therefore errors creep in over time, and in so doing degrade the overall accuracy of the INS unit.
The way around this small problem with INS, was to have the raw INS data fed into a navigation computer, and in turn, that unit uses an external source (for example DME/DME triangulation) to update the computed aircraft position.
The first widebody type to use this sort of latter described arrangement (a flight management system, IE: FMS, which also included full time engine thrust management in the mix) was...the Lockheed TriStar.