A diagnosis of "autistic disorder" is made in accordance with the Diagnostic and Statistical Manual of the American Psychiatric Association, a diagnostic handbook which is now in its fourth revision (commonly referred to as "DSM-IV"). Several related disorders are grouped under the broader heading of "Pervasive Developmental Disorder" or PDD. The DSM-IV lists criteria to be met for each specific diagnosis under the category of PDD. Diagnosis is made when a specified number of characteristics listed in the DSM-IV are present. Diagnostic evaluations are based on the presence of specific behaviors indicated by observation and through consultation with parents and other caregivers, and should be made by an experienced, trained team. Thus, when professionals or parents refer to "different types of autism", they are usually distinguishing autism from one of the other pervasive developmental disorders.

The five Pervasive Developmental Disorders listed in DSM-IV are: Autistic Disorder, Asperger's Disorder, Pervasive Developmental Disorder-Not Otherwise Specified (or "PDD-NOS"), Rett's Disorder and Childhood Disintegrative Disorder. Autistic Disorder, Asperger's Disorder, and PDD-NOS are generally considered "Autism Spectrum Disorders", or "ASDs".

There is no standard "type" or "typical" person with autism. Parents may hear different terms used to describe individuals within the spectrum of autism, such as: autistic-like, autistic tendencies, autistic spectrum, high-functioning autism, low-functioning autism, etc. More important to understand is, whatever the technical or descriptive diagnostic label, these children and adults can learn and gain new skills throughout their entire lives, provided that they are provided with the appropriate education and treatment.