Advisory Circulars are informational documents produced by the Federal Aviation Administration to inform and guide institutions, operations, and individuals within the aviation industry, as well as the general public. Advisory Circulars are intended to be informative in nature and not regulatory; however, many times they describe actions or advice that the FAA expects to be implemented or followed.
ACs can be distributed by the FAA to an audience of pilots, mechanics, operators, airport managers, manufacturers, and the general public. The subject of the advisory circular typically involves aircraft, airports, flight schools, pilots, operations, or maintainers. Advisory circulars can be directional,informational or descriptional. They often describe how the FAA wants things should be done, best practices for operations, or clarification of a new regulation.
The FAA issues advisory circulars for many reasons. Standardization is one very popular reason. As there are often many ways to interpret regulations and many ways to implement a specific operation, an advisory circular can offer specific guidelines for the aviation industry when the regulations or requirements are otherwise vague.
Advisory Circulars can also be distributed as a safety precaution, such as to notify pilots of an equipment malfunction or a rule change, like the pilot fatigue and duty requirement regulations that were recently changed.
A very recent example of an advisory circular is AC-No 120-76A, which involves guidelines for the use of electronic flight bags and other electronic on-board equipment. The introduction of the iPad and other tablet devices made this advisory circular necessary, as pilots weren't sure what formats were legal to use during different types of flying environments.
You can see full list of current advisory circulars on the FAA's website, or by clicking here.