Applied Behavioral Analysis (ABA) is a form of treatment for autism that involves observation of behaviors and intensive behavioral interventions meant to bring positive change over a period of time.
It is considered a safe and effective treatment for autism and "has been endorsed by a number of state and federal agencies, including the U.S. Surgeon General and the New York State Department of Health." 
There is no one program that is ABA. Programs instead are customized to each child or adult based on their abilities, skills, interests and needs. Each program, however, should begin with an assessment, or an observation of current behaviors. While ABA is also used on adults with autism, in this post we will refer to "child" as the individual receiving the therapy.
Analysis of Behavior
Trained therapists work with individuals to determine their current abilities by using the ABC model:
A - Antecedent - A directive is given. This could be "pick up the block." It is a request for an action to be performed.
B - Behavior - Your child's behavior is observed. Did he comply with the request, did he refuse to comply or was there no response.
C - Consequence - The result of the action by the therapist. Consequences can be positive or negative. For example, if your child picked up the block, the therapist may give him a treat or praise him. If your child did not comply with the request, the consequence might be that he does not receive a treat.
Once the therapist does a complete evaluation, a trained therapist will work with your child and family to develop goals and target behaviors. Each skill that is to be taught is broken down into steps, for example, ABA for speech may start with making sounds and include steps that lead up to holding a conversation.
Periodic reviews and assessments will continue throughout treatment to measure your child's progress and the therapy will be adjusted as needed.
Types of Interventions
ABA uses a variety of different interventions and techniques:
Chaining - Breaking down a task into the simplest steps, for example if the therapist is working on the task of getting dressed, the first step might be opening the drawer of the dresser. Once the chain of events is determined, the therapist works with your child until the first step is learned and then moves on to the next step.
Modeling - The therapist demonstrates the desired behavior.
Prompting - Encouraging your child to perform the desired behavior. Since the goal of ABA is independence, prompts are as minimal as possible. For example, if the therapist is working with your child to get dressed, she may point to the drawer or say "Open the drawer."
Fading - As your child learns a new skill, prompts are reduced. For example, your child may have initially needed physical help to open the dresser drawer, then the therapist may have directed him to open the drawer until your child is able to complete the task without any prompts.
Shaping - This technique slowly takes an undesired behavior and changes it to a desired behavior. For example, suppose each time your child picks up a toy he throws it. The therapist may work to have him pick up a toy and put it down gently. In the beginning, the therapist may need to physically take your child's hand, lift the toy and then put it down, praising your child for putting the toy down. Slowly your child will learn the preferred way to pick up the toy and put it down in front of him.
Reinforcement - ADA uses positive reinforcement as a basis for teaching skills. Praise, a small treat or any other motivating action/item is used to help encourage the desired behavior.
Generalization - Many skills are learned in a controlled environment, such as at a play center or at a table. Once these skills are learned, they are expanded into different environments. For example, your child might learn colors through play but then that knowledge is used in different areas of the house, at the grocery store or while taking a walk outside.
These ABA techniques are used by the therapist and taught to parents and caregivers so that the strategies are used and incorporated into daily life. Therapists will also look for child-initiated activities to reinforce learning, for example, if your child takes a toy out to play, the therapist will use the opportunity to reinforce other skills.
Length of Treatment
Because each treatment model is customized to the needs of the individual, there is no specific length of treatment, however, ABA is an intensive treatment program and most experts believe that the program should be carried out for a minimum of 25 hours per week for a year.
In the original ABA program, created by Ivar Lovaas in 1987, ABA techniques were used for 40 hours per week for 2 years. While there is no exact length of treatment, ideally, the principles and techniques of ABA should be incorporated into daily life and therefore used every day.
 "Applied Behavior Analysis (ABA)," Date Unknown, Staff Writer, Autism Speaks
"Applied Behaviour Analysis," 2007, Lara Pullen, Ph.D., Autism Canada Foundation
"Introduction to Applied Behavior Analysis," Date Unkown, Barry K. Morris, Autism -Help.org: Autism Spectrum Disorder Fact Sheets