1. Autistic Disorder is the formal term for what is more commonly called "autism".  The disorder is characterized by a pattern of severe impairments in three areas:  difficulties interacting with others in a reciprocal way, impaired language and communication skills and a repetitive and restricted range of interests and activities.  The symptoms emerge before the age of three, but can change over time and vary widely from one child to another.
2. Asperger's Disorder describes children who have average intelligence and who do not have a history of delayed language development.  They do, however, have social impairments and restricted, repetitive interests.  Their social interactions can be awkward and one-sided and they often have difficulty understanding the perspectives of others. They may talk unceasingly about some object or topic of fascination, not understanding the give-and-take of conversation or the art of listening.  This diagnosis is used only when the impairments are severe and sustained and interfere with the child's functioning at home, school or in the community.  Many times the symptoms of Asperger's are not evident until the child begins school.
3. Rett's Disorder is a very rare disorder, mostly affecting girls exclusively.  Children with this disorder develop normally and on schedule through early infancy.  But sometime between six and eighteen months, they gradually begin to lose skills in different areas of functioning.  Children who had been talking, stop.  Their ability to interact with others decreases.  They also lost the use of their hands to hold and manipulate objects and begin to show repetitive hand movements such as hand rubbing, clasping or wringing.  During this phase of their regression, they show symptoms similar to those seen in Autistic Disorder.  The symptoms of Rett's Disorder are progressive and worsen over time.
4. Childhood Disintegrative Disorder is a very rare condition that involves a significant regression in skills in children who have had typical development for the first two years of live.  Between the ages of two and ten, children with CDD lose some or all of the skills they have already developed in areas that include language, social skills, play and motor skills.  Following this period of regression, their behavior stabilized, though children usually have severe mental retardation and may also have seizures.
5. Pervasive Developmental Disorder Not Otherwise Specified (PDDNOS) is what's called a "diagnosis of exclusion."  It is used only when a child demonstrates symptoms of PDD that do not fit the criteria specified for one of the other disorders in this category.  Specifically, the diagnosis of PDDNOS is used for children who are impaired in their social interactions along with either an impairment in the development of language and communication skills or a pattern of restricted or repetitive behaviors and activities.  A child may receive a diagnosis of PDDNOS rather than Autistic Disorder if he or she does not show all of the symptoms required for a diagnosis of Autistic Disorder or if his symptoms are milder in nature.

These disorders differ from each other on features that include prevalence, severity and the way in which their symptoms appear and progress.  But, in my opinion, they all have one thing in common, they all suck.