Like the name suggest, the system provides Heading and attitude info to the aircraft ADI and HSI. You can view the AHRS as an integrated VG/DG system or a poor-man IRS. The flexdisc gyrometers you mentioned must be specific to the AHRS you are thinking of.
Basically a typical AHRS contains 2 (or 3) gyroscopes, 3-accelerometers. A gyro picks up rotation perpendicular to its (spin) axis. Each accelerometer picks up acceleration (change in velocity) along its axis. It can detect the earth gravitational field. Together these info are processed by the AHRS computer to give you aircraft attitude (pitch, roll, yaw) and Heading. For magnetic information, flux valves used to be really popular but they are now mostly replaced by magnetic variation tables that computers applied to computed true headings to get magnetic headings.