The different kinds of autism disorders form a range of developmental disorders, which together make up the autism spectrum. All types of autism disorders are developmental disorders marked by an aversion to social interaction and sometimes rigid or repetitive behavior, among other things. The autism spectrum is a scale that accommodates cases of differing severity and symptoms. For example, some disorders distinguish between different levels of language learning or the age of onset. While terms like "high-functioning autism" are sometimes heard, such terminology is relative and has no fixed definition.
As autism is a developmental disorder, all kinds of autism are marked by the failure to fully develop certain skills. Autism disorders have in common a lack of typical social development and communication, and fixed interests or behavior that may appear obsessive. Social symptoms can include not responding to one's own name when called, a lack of sustained eye contact, and an inability to interpret what others are thinking or feeling. It is important to note that these symptoms can vary greatly in severity, as well as in combination. For instance, an autistic person may have fairly typical language development but nevertheless have great difficulty carrying on conversations.
In the United States, the Diagnostic and Statistical Manual, published by the American Psychiatric Association, contains all the mental health disorders the Association recognizes. It is better known as the DSM with a number designating its current edition, because it is subject to periodic revision. In the U.S, it is considered the definitive reference for mental health disorders. It recognizes a handful of autism disorders, including Autistic Disorder, Rett's Disorder, Asperger's Disorder, Childhood Disintegrative Disorder, and the "atypical" Pervasive Developmental Disorder.
Autistic disorder is often simply referred to as autism. In addition to commonly lacking social development and communication, autistic people often have very specific interests. Autistic children might prefer to line up toys instead of the imaginative play typical of developing children, or prefer being alone instead of playing with others. While these terms are relative, a high-functioning autistic person could have above-average language skills while a low-functioning autistic person could exhibit mental retardation.
Other autism disorders include Asperger's disorder, which is marked by capable language skills and sometimes above-average intelligence. People with Asperger's display other autistic behavior like impaired social development, though usually to a somewhat lower degree. Rett's disorder is a specific genetic disorder that usually occurs in girls, and is marked by normal development that slows or regresses at one to two years of age.
Childhood Disintegrative Disorder (CDD) is a rare disorder that also includes regressive development, usually in social development occurring at three to five years of age. Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NSO) is also called atypical autism. This could include, for instance, autistic symptoms of lower severity, or only one specific autistic symptom but no others.