this is a very brief explanation, but a good start:
Autism Spectrum Disorder a.k.a. Pervasive Development Disorder "PDD" includes the following
1)aspbergers syndrome
2)Retts syndrome (rare)
3)Childhood Disintegrative Disorder (rare)
4)autism (must have six pervasive symptoms over social, language, and behavior domains)
5)PDD-NOS- which means Pervasive Developmental Disorder Not Otherwise Specificied.. so the child doesn't meet the diagnositic criteria, which are very specific, for the other four categories, but has atypical aspects of social, language, and behavior.
High Functioning Autism is an informal term and usually means the kids who do not meet the aspergers criteria (which you can find easily on the web) but are on the autism spectrum. Many officially have the "PDD-NOS" label. They differ from aspbergers in one major way and that is "aspies" must have extremely strong VERBAL abilities relative to other areas, while kids with HFA do not.
THen there is the gifted part in the mix and that is addressed in teh books Grinity recommends.
irene
