Autism Spectrum Disorders (ASD), also known as Pervasive Developmental Disorders (PDDs), cause severe and pervasive impairment in thinking, feeling, language, and the ability to relate to others. These disorders are usually first diagnosed in early childhood and range from a severe form, called autistic disorder, through pervasive development disorder not otherwise specified (PDD-NOS), to a much milder form, Asperger syndrome.

Source: NISH autism link

Asperger syndrome is an autism spectrum disorder, a group of conditions that include autism and other disorders with similar symptoms, such as problems with language and communication, and repetitive or restrictive patterns of thoughts and behavior.

People with Asperger syndrome have autism-like problems in areas of social interaction and communication, but have normal intelligence and verbal skills.  Asperger syndrome is usually thought to be the mildest of the autism spectrum disorders.

Source: asperger_syndrome