INS (Inertial Navigation System) is a generic term for whatever tells you where you are with respect to some reference frame (no system uses a truly inertial reference frame, but that's another rant!). Swap "System" for "Unit" and you've got an INU - an equally generic term. Any "strap down" navigation system that uses rate gyros and accelerometers takes some amount of time to "align" to ensure the attitude and position that it reports is as accurate as it can be (or at least, within spec). Flight Gear has a pretty good wiki-style article on INS systems here http://wiki.flightgear.org/Inertial_Navigation_System
The Ka-50's doppler system (if that's what you're referring to by "DNS") determines the Ka-50's velocity relative to the ground, but would not be sufficient for a complete navigation system. I believe this system's output is used solely by the auto hover system (and the HUD, which reports the measured ground-speed in the upper left).