What are PDDs and ASDs?

The term PDD or Pervasive Developmental Disorder is used to describe certain problems in development. PDDs are called "spectrum" disorders because each child has symptoms that differ in intensity, ranging from mild to quite severe. All children with PDD, however, have some degree of difficulty in the following areas: 

Social skills: Such as sharing emotions, understanding how people are feeling, expressing empathy or having a conversation.
Communication: Both spoken and unspoken, such as pointing, gesturing and making eye contact.
Behaviors or interests: Such as repeating words or actions, playing with things in an unusual way (spinning objects, lining up toys) or insisting on following rigid routines or schedules

There are five PDDs: Autistic Disorder, Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS), Asperger Syndrome and the less frequently occurring disorders: Rett Syndrome and Childhood Disintegrative Disorder (CDD)

Autism Spectrum Disorders:

Autistic Disorder or Autism is characterized by difficulties in all three areas (communication, social skills and behavior - see above).
Asperger syndrome is characterized by relatively good basic language skills and difficulties in social interactions and conversation. A child with Asperger Syndrome may also have intense or restricted interests and/or challenging behaviors.
PDD-NOS describes a person who meets some but not all the criteria for autism or has milder symptoms in one or more areas.

Your child's specialists might use slightly different terms to describe your child. But even when professionals don't always use these terms the same way, they usually agree that a child fits under the broader ASD diagnosis. 

Less frequent Pervasive Developmental Disorders:

Rett Syndrome is a developmental disorder that begins with normal early development and is followed by loss of motor skills, particularly hand use, and distinct hand movements, such as wringing, regression in skills and slowed growth.  
Childhood Disintegrative Disorder begins with normal development until the age of 3 or 4 and is followed by severe loss of communication, social, play and motor skills.