I designed the (hopefully) simple graphic below to help people understand the family of Autism Spectrum Disorders — as outlined in the DSM-IV — and how they relate to one another (click on the image to see a larger version). I did not include Rett's Disorder or Childhood Disintegrative Disorder, as they usually receive their own diagnoses apart from an ASD.
As you can see, much of the confusion comes from the duplicative use of terms like PDD and ASD at both the category and sub-category levels.
It's important to note that the just-approved DSM-V subsumes both PDD-NOS and Aspergers into one category called, quite simply, Autism. In addition, a new category called Social Communication Disorder has been created to hopefully account for children who don't meet the new ASD criteria but still have deficits in the key areas of communication and social skills.
Let me know what you think on Twitter: @1asddad