Autism is part of the five pervasive developmental disorders (PDD). These are characterized by:

• abnormalities of social interactions and communication
• restricted interests
• highly repetitive behavior

Autism has a wide range of severity and symptoms that is often used to classify the Autism Spectrum disorders. Each of the syndromes under ASD is different from the other. For example, people with Asperger syndrome have no substantial delay in language development.

Autism itself is often called ''autistic disorder'', ''childhood autism'', or ''infantile autism''. In some individuals autism may be silent or manifest only as a mental disability while in others there are repetitive movements like hand flapping and rocking.

Some autistic individuals may be normal in all factors of life except for being awkward socially. They may have narrowly focused interests, and verbose, pedantic communication. Boundaries between diagnostic categories are necessarily somewhat arbitrary because of the overlapping and myriad of features.

Autism diagnosis

Autism can normally be diagnosed in children at around the age of two. However, it can be difficult to diagnose as the symptoms will often only become more noticeable as they get older. Some people with ASD grow up without ever being diagnosed.

Types of ASD

Being a range of disorders autism includes a wide variety of disorders of varying severity. Some of the types of ASD include:

• Autistic disorder, sometimes known as "classic autism". This manifests as significant language delays, social and communication challenges, and unusual behaviors. There may be additional learning difficulties and below-average intelligence as well.
• Asperger syndrome – Symptoms are milder than classic autism. There are social challenges and unusual behaviors. There may be typically no language problems or intellectual disability. However, some areas of language development may be affected. They may typically have problems with understanding humor or figures of speech. Some children have particular skills in areas that require logic, memory and creativity, such as maths, computer science and music.
• Pervasive developmental disorder – not otherwise specified (PDD-NOS), also known as "atypical autism" – these individuals meet some of the criteria for autistic disorder or Asperger syndrome, but not all. Symptoms may be fewer and milder. There may be social and communication challenges.

Children with ASD may concomitantly also have other problems such as attention deficit hyperactivity disorder (ADHD), Tourette's syndrome or other tic disorders, dyspraxia (developmental co-ordination disorder), epilepsies etc.

Autism can also be divided into syndromal and non-syndromal autism. Syndromal autism is associated with severe or profound mental retardation or a congenital features such as tuberous sclerosis. For example, those with Asperger syndrome. Aspergers syndrome, however, is different from other autism syndromes as these individuals tend to perform better cognitively than those with autism.

Autism may also be of the regressive type. In these children (for it is seen commonly in children), the diagnosis of autism is made on the basis of loss of language or social skills, as opposed to a failure to make progress, typically from 15 to 30 months of age. This could be a specific subtype.

Other tests for autism subtypes

Apart from diagnosis of individual subtypes by interview based tests, other tests are also useful. Newer technologies such as fMRI can help identify biologically relevant subtypes that can be viewed on brain scans, to help further neurogenetic studies of autism.

Genes are being studied as well to define the subtypes of autism. For example ''Type 1 autism'' denotes rare autism cases that test positive for a mutation in the CNTNAP2 gene.