I've worked on and used INS's for some 28yrs now, in both military and civil jets, and I must admit, I understand them differently from the above posts. So, for what it's worth, here's my two pennorth:
The inertial platform senses accelerations from a given state or rest or movement. The associated computer integrates the accelerations and provides velocity and distance data. There is obviously a lot more to it, but basically, all the inertial system will give you is those values.
Using other aircraft inputs, and variations in inertial data. the cleverer systems can give you position on the earth's surface.
Now using the inertial position, inertial direction of travel and inertial distance covered and then comparing them with Air Data, e.g. true airspeed, compas direction, etc, the inertial system will provide drift, groundspeed and wind data.
More simply, the INS knows where you are going and how fast you are going. The ASI provides the TAS and the compas provides the direction you are pointing in. Put the lot together and you can work out wind speed and drift. On it's own, with pure inertial data, it is impossible to work out drift and wind speed.
Hope this helps,
A