A basic IMU (Intertial Measurement Unit) generally provides raw sensor data, whereas an AHRS takes this data one step further, converting it into heading or direction in degrees, converting the raw altitude data into standard units like feet or meters, etc.
To help you get started designing your own AHRS system, or just tp help convert raw sensor data into useful numbers that you can relate to the real world, we've created a sample AHRS sketch for the Adafruit 10-DOF IMU, Adafruit 9-DOF IMU, and Adafruit_LSM9DS0 breakouts.