The learning is broken down into very small, or 'discrete' steps and taught using repetition, positive reinforcement, and whatever degree of prompting is necessary for success. Typically, children would participate in this kind of a program for 30 - 40 hours a week, and follow a curriculum tailored to their needs and level of development. 

Often these programs are started in the home, with the family hiring and training a team of 'workers' or 'therapists' to do the teaching. The curriculum is frequently designed and supervised by a Behavioural Psychologist, although there are many books and web sites that provide curriculum so that parents can create their own program. 

Check out these web sites for more information:

Autism Research Institute
Geneva Centre for Autism
P•E•A•CH - Parents for the Early Intervention of Autism in Children
FEAT (Families For Early Autism Treatment)