AAC refers to augmentative and alternative communication. It includes all forms of communication, besides oral speech, that are used to express thoughts, needs, wants, and ideas. Examples range from simple facial expressions and gestures to the used of sign language and special augmentative electronic aids. There are two basice types of AAC:

• Aided devices require the use of some additional object within the environment (paper and pencil, picture board, etc.)
• Unaided devices rely on the user's body to convey messages (gestures, sign language, etc.)

Here are some links to learn more about AAC:
http://sitemaker.umich.edu/356.kobza/pecs__picture_exchange_communication_system
http://www.aacinstitute.org
http://www.resna.org/
http://www.asha.org/public/speech/disorders/AACSLPbenefits.htm
http://www.asha.org/public/speech/disorders/AAC-Glossary/