What is the difference between IMU (Inertial Measurement Unit) and AHRS (Attitude and Heading Reference System)?
An INS calculates and updates the vehicle's position (latitude and longitude), alongside the orientation. It needs to be initialized on ground, with the aircraft completely still.

An AHRS does not record/update the position. It outputs real time orientation (attitude and heading) only. It thus can be initialized in flight.

(Image source)

Image shows the INS panel of a Boeing 747-400, notice the ATT (Attitude) option, that's an AHRS-only mode. NAV includes both AHRS and navigation.

ATT can be initiated in-flight, it requires a manual entry for the heading while flying straight and level. So in other words, an INS includes AHRS-functionality.




  Sidenote on terminology: INS nowadays is predominantly used to describe an Inertial Platform, Inertial Reference System (IRS), and Inertial Reference Unit (IRU/IMU); and often used to describe all three of these units (as one system) typically installed on a single aircraft.