Some people classify autism spectrum disorders into subtypes by their presumed causes, but because science hasn't yet resolved the causes, this practice is controversial and not widely accepted. But you can look at autism as a spectrum ranging from severe to light.
People with severe autism might be greatly disabled, whereas those with high-functioning autism (HFA) and Asperger Syndrome may be affected so slightly that their autism doesn't play a major role in their lives.
Some researchers include Attention Deficit/Hyperactivity Disorder (AD/HD) on a broader autism spectrum because of its similarities to Asperger's; others claim AD/HD is a clinically distinct disorder that may occur simultaneously with autism.
On the other hand, some researchers don't consider Asperger Syndrome to be autism at all; others consider it the same as high-functioning autism.
Severe (or "classic") autism
Sometimes referred to as Kanner's Syndrome, severe autism is the classic type of autism that books and films often portray to great dramatic effect. You may also hear it called infantile autism, childhood autism, or simply autism disorder. Individuals with the classic type of autism may have most of the 12 symptoms of autism, or they may have only a few obvious ones. Some of the symptoms can be so debilitating — like a lack of functional communication — and the sensory issues so severe that the afflicted can barely stand to remain in their own skin. Other symptoms may be mild; a person may have good verbal communication skills but is unable to understand pragmatics, or the meaning "between the words."
Language develops late or not at all in people with Kanner's Syndrome, which is the main distinction between classic autism and Asperger's, as of this writing. Dr. Temple Grandin, a professor of animal science at Colorado State University who lectures and writes frequently on autism, and Kathy Grant, a political science graduate and autism advocate who has chronicled her sensory sensitivities, are some famous examples of high-functioning people with classic autism.
PDD and PDD-NOS
Pervasive Developmental Disorder (PDD) is the category containing autism, Asperger Syndrome, Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS), Childhood Disintegrative Disorder, and Rett Syndrome. PDD-NOS is a catchall diagnosis for people having most but not all the characteristics of autism. People with these disorders are often described as "autistic-like" or developmentally delayed with autism symptoms; recently, experts have put them on the autism spectrum.
Although classified under the Pervasive Developmental Disorders, Rett Syndrome has identifiable physical differences such as reduced muscle tone (causing the child to seem "floppy") and stereotyped hand movements such as wringing or waving. Additionally, Rett Syndrome produces an identifiable chromosomal difference.
People with PDD-NOS need some special services but don't fit the behavioral criteria for any of the other categories of autism. Maybe the person lacks meaningful speech or has stereotypical movements such as hand flapping or rocking, but he or she doesn't have enough of the symptoms to fill the symptom bucket and be considered autistic. This lack of a label may make it tougher to get services, because the autism label — although considered stigmatizing by some people — does generally get you educational services in most places.
Because autism is a spectrum disorder without a clear dividing line, some people fall just on the other side of the line — they don't qualify for autism disorder because of late onset of symptoms, or they don't have enough behavioral symptoms. However, their disorder significantly impacts daily functioning on a regular basis.
Asperger Syndrome
Individuals with Asperger Syndrome range from people who may be considered a little eccentric to people who have serious difficulties socially, educationally, and professionally because they lack basic understanding of human interactions. People in the latter group often have to learn by rote things that other people consider common sense, such as how to read facial expressions, tones of voice (like sarcasm), and verbal expressions (such as "raining cats and dogs").
Many people with Asperger's have brilliant intellects yet are naïve and easily taken advantage of because they interpret situations at face value and miss social cues. Many undiagnosed individuals feel different from others but don't know why. Generally, "Aspies" lack common emotional responses and must learn appropriate social skills to function within society, but they're typically considered high functioning and may never be diagnosed at all. No obvious language delay comes with Asperger Syndrome; however, language tends to develop in a unique manner. Professionals dispute whether Asperger's should even be considered a disorder. People affected don't show the same delays in cognitive development or curiosity about their environment that people with classic autism do in childhood.
People with Asperger's don't lack feelings; their brains just function in such a way that they have trouble accessing and expressing feelings to others in a traditional manner.