Applied Behavior Analysis (ABA) is a psychological science of reinforcing behavior, which used to be called Behavior Modification, and is sometimes synonymous with the ABA subtype, Positive Behavior Support (PBS). Most people inaccurately think that ABA refers specifically to treating autistic children. ABA subtypes used for autistic children include Early Intensive Behavioral Intervention (EIBI) and, for older children, the Lovaas Model, which is named after its inventor, Dr. Ivar Lovaas. Pivotal Response Treatment (PRT), formery called the Natural Langauge Paradigm, is a play-based teaching methodology that utilizes the science of PBS to modify behavior.
EIBI begins with an intervention that focuses on, what is known as, Discrete Trial Teaching (DTT), which is a one-to-one interaction between the child and a trained therapist, who creates a highly structured, individualized and systematic learning environment. The process requires a significant investment in time to be effective; Lovaas recommends 40 hours a week of intensive therapy for two years. 
In the intervention, skills are broken down into small, achievable tasks. Every task then consists of three parts: The therapist asks the child to perform a specific action; the child responds; and the therapist reacts to the child's response.
When the child successfully completes a task, the therapist gives the child praise and reinforcement. The rewards for successful completion may be small bites of food, playing with the child's favorite toy, hugs and tickles. Gradually, external rewards like food and hugs are replaced with verbal praise and social reinforcers. Aggressive and self-stimulating behaviors are redirected into more socially acceptable responses.
Eventually, it is reccomended to further "generalize" what the child has learned with EIBI-based methods using child instructed, play-type methods in more natural settings such as Natural Enviroment Teaching (NET, a generic term for Incidental Teaching) and Fluency-Based Instruction.
Research shows that utilizing both structured and play-based EIBI methods yield the most effective results. Moreover, 42% fully recover in that they no longer have deficits in social interaction, sterotyped behaviors or interests, or in verbal and non-verbal communication. They also no longer show signs in the core deficit of autism spectrum disorders (ASD): mind-blindedness (or Theory of Mind).
A study completed in April 2007 by the UK foundation Research Autism found that 25 hours a week of therapy could significantly raise IQ scores.  According to news service NewsWales, "IQ increased for two thirds of the children receiving the early intervention and very substantially for more than a quarter of them. For example, one child moved from an IQ of 30 up to 70; another from an IQ of 72 to 115. Most of the population of the UK has an IQ of between 85 and 115."