Each satellite manufacturer has its own configuration so this is a basic description of the concept of the Attitude Control System on GEO Satellites.  The ACS hardware can be broken down to 2 basic categories, sensors and actuators.  Sensors collect measurements and information in reference to the spacecraft’s axises. This information is converted into error signals used to control each axis.  Sensors include but are not limited to, Earth Sensors, Sun Sensors and Gyros. Actuators allow control of movement in the axis. Typical hardware used as actuators includes but is not limited to,  Momentum Wheel Assemblies, Reaction Wheel Assemblies, Magnetic Torque  Rods and Reaction Control Thrusters. Antenna tracking systems and Solar Array systems have also been used as actuators, but are not part of the basic set of actuators normally used.  Although they are predominately used in 2-Axis GEO Satellites.

A third category, being the ACS software is normally referred to when talking about Attitude Determination and Control System (ADCS). The ACS software will collect the error signals from the sensors and convert them into control signals to be applied to the actuators to maintain control of each axis.