What is Picture Exchange Communication System (PECS)?
It is a system designed for individuals who cannot or are having difficulty communicating verbally. Pictures cards are used with a communication board and/or binder allowing a child to express their needs, desires and feelings. 
This system:
• Teaches communication and interaction
• Teaches proper sequencing of words to form sentences
• Makes your autistic child more independent
• Is portable and useful in virtually ALL environments

PECS has six phases of implementation.
A speech therapist can be helpful in ensuring the system is correctly implemented and used. 

Phase 1 How to Communicate

•  Students learn to exchange single pictures for items or activities they really want.
  

Phase 2 Distance and Persistence

• Still using single pictures, students learn to generalize this new skill by using it in different places, with different people and across distances. They are also taught to be more persistent communicators.
  

Phase 3 Picture Discrimination

• Students learn to select from two or more pictures to ask for their favorite things. These are placed in a communication book—a ring binder with Velcro® strips where pictures are stored and easily removed for communication.
  

Phase 4 Sentence Structure

• Students learn to construct simple sentences on a detachable sentence strip using an “I want” picture followed by a picture of the item being requested.
  
• Attributes and Language Expansion
  Students learn to expand their sentences by adding adjectives, verbs and prepositions.
  

Phase 5 Answering Questions

• Students learn to use PECS to answer the question, “What do you want?”.
  

Phase 6 Commenting

• Now students are taught to comment in response to questions such as, “What do you see?”, “What do you hear?” and “What is it?”. They learn to make up sentences starting with “I see”, “I hear”, “I feel”, “It is a”, etc.
  

If your son or daughter is non-verbal ask your speech therapist about PECS and if it's right for your child. Depending on the child's age and comprehension a more advanced communication device may be more appropriate.


Sources: http://www.mahalo.com/how-to-treat-autism/
http://www.autismepicenter.com
http://www.pecs.com
http://www.pecsusa.com/pecs.php