In simple terms, a SAS will give you short term rate damping but no long term attitude hold. A basic AP will give you long term Attitude retention e.g. you deflect the cyclic aft to raise the nose 10 degrees, then release it, and the a/c will return to the original pitch attitude.
A more sophisticated AP will give you 'outer loop', or 'upper mode' functions whereby it will couple to external references, such as Altitude, IAS, HDG, NAV and/or ILS (the number of discretes will depend on the sophistication of the AP). If very sophisticated, it will have full control of all 4 control axes and may allow transitions to the hover and other such SAR functions.