The first three of these disorders are commonly called the autism spectrum disorders; the last two disorders are much rarer, and are sometimes placed in the autism spectrum and sometimes not.

There are three diagnoses that are currently classified as Autism Spectrum Disorders (ASDs). They are:
• Autistic Disorder, sometimes known as classic autism or childhood autism
• Asperger syndrome, sometimes known as Asperger's Disorder
• Pervasive Developmental Disorder – Not Otherwise Specified (PDD-NOS)

All three share characteristics, sometimes known as the 'triad of impairments', namely communication difficulties, social interaction difficulties and repetitive behaviours/restricted interests. Children diagnosed with an ASD will have some kind of difficulty in all three areas, though these vary significantly from person to person.

Retts Disorder – is a rare form of autism. Rett's syndrome affects the way the brain develops and occurs almost exclusively in girls. Babies develop normally at first but when the infant reaches around 18 months, the child starts showing problems with motor skills which may affect their ability to use their hands, feet, affect speaking and chewing and difficulty breathing. Only about one of every 10,000 – 15,000 girls will develop this disorder and though there is no cure, there are a number of treatments used to help the child and their family function on a day to day basis.

Childhood disintegrative disorder sometimes called Heller Syndrome - is a condition in which children develop normally through age 3 or 4. Then, over a few months, children lose language, motor, social, and other skills that they already learned.

In Autism Spectrum Disorder(ASD), we see Autism, Asperger And PDD-NOS.

This pictures illustrates how each three classified.
PDD