Dear Marie,
Autism is one type od Pervasive Developmental Disorder. In other words, PDD is a category of emotional disorders, and Autism is one particular type in this broad category.
Autism is characterized by qualitative impairment in social interaction, qualitative impairments in communication, and by restricted repetitive and stereotyped patterns of behavior, interests and activities. Prior to the age of three, delays or abnormal functioning is displayed in social interaction, language as used in social communication or in symbolic or imaginative play.