High-functioning autism is characterized by features very similar to those of Asperger syndrome. The defining characteristic most widely recognized by current psychologists and doctors is a significant delay in the development of early speech and language skills before the age of 3 years.[2] The diagnosis criteria of Asperger syndrome exclude a general language delay.[5]

Further differences in features between people with high-functioning autism and those with Asperger syndrome include the following:[2][6][7][8]

People with HFA have a lower verbal intelligence quotient
Better visual/spatial skills (higher Performance IQ) than people with Asperger syndrome
Less deviating locomotion than people with Asperger syndrome
People with HFA more often have problems functioning independently
Curiosity and interest for many different things, in contrast to people with Asperger syndrome
People with Asperger syndrome are better at empathizing with another
The male to female ratio of 4:1 for HFA is much smaller than that of Asperger syndrome

Understanding Low Functioning Autism
Low functioning, rather than high functioning autism, is a form in which people have little awareness of those around them. Individuals can have multiple impairments in virtually all areas of development. Children who have this form of autism display it differently from those at the other end of the spectrum.

They often have odd behaviors, rituals and gestures that are apparent to others.
They are more likely to self injury.
They have few expectations.
Many have severe memory impairment, unable to remember the names of people or things.
Many suffer from epilepsy.
Most children display severe disabilities and have receptive and expressive language skills, which are severely limited.
What is unfortunate is most autism research done today focuses on high functioning autism. Because of this, doctors do not have as much information or as many treatment options available to children at the opposite end of the spectrum. It is important to keep in mind that children labeled as low functioning autistic children can see improvements in their language. They can improve socially and may be able to move towards high functioning autism. This does not happen to all children, but no parent should lose hope that their child cannot see some improvement.

Heres a good blog I just found today, she actually is on MAT as well...
http://changedforgoodautism.blogspot.com/2014/0...

Our developmental pediatrician told us that 40% of those with a "classic" autism diagnosis (under the DSM-IV) also have mental retardation. Those folks are considered to have LFA. The other 60% have an IQ that is in the average-to-above-average range. They are considered to have HFA. A person with HFA can either be moderately or mildly impaired. My DD's original diagnosis was HFA, moderate impairment. She has since improved a lot in her functioning level and has moved towards the "mild" end of the spectrum. I haven't had her reevaluated so I'm not sure whether at this point she would officially be considered mildly impaired. But her school just moved her up from their mild ASD class to a class for kids with language disabilities.

@A MyAutismTeam Member, I agree with you but in the clinical sense...

So, as stated, I will stick to my favorite answer, "It depends on whom you are comparing him to." A bit tongue in cheek but I have always felt it fit best. :o)*