Autistic Disorder (also called "classic" autism)
This is what most people think of when hearing the word "autism." People with autistic disorder usually have significant language delays, social and communication challenges, and unusual behaviors and interests. Many people with autistic disorder also have an intellectual disability.
Centers for Disease Control and Prevention. (2012, March 12). Autism Spectrum Disorders (ASDs).
People with Asperger syndrome usually have some milder symptoms of autistic disorder. They might have social challenges and unusual behaviors and interests. However, they typically do not have problems with language or intellectual disability.
Pervasive Developmental Disorder – Not Otherwise Specified (PDD-NOS; also called "atypical autism")
People who meet some of the criteria for autistic disorder or Asperger syndrome, but not all, may be diagnosed with PDD-NOS. People with PDD-NOS usually have fewer and milder symptoms than those with autistic disorder. The symptoms might cause only social and communication challenges.