Autistic Disorder is what most people think of at hearing the word "Autism". People with Autism typically have social and communication difficulties. People with this type of autism tend to have intellectual disabilities and signifigand language delays.
People with Asperger Syndrome typically have milder signs of Autism and might have unusual behavior or interests. They usually don't have problems with language or education.
Pervasive Developmental disorder
People who meet some of the requirements for Aspergers, but not all may be diagnosed with PDD, the symptoms are milder than that of Aspergers and mostly suffer only from small social or communication problems.