Emotional disorders of childhood are organized in a number of categories. For example, some conditions are Anxiety Disorders; some conditions are Disruptive Behavior Disorders, etc. Within each category are specific conditions. One of the major categories is Pervasive Developmental Disorders, and within this category are a number of specific conditions (e.g., Autistic Disorder, Asperger's Disorder, Rett's Disorder). All the types of Pervasive Developmental Disorder are typified by developmental delays/deficiencies in communication skills and socialization skills, as well as by a number of other characteristic (e.g., restricted range of interests, repetitive and stereotyped movements). The 'umbrella term' autism spectrum can refer in a general way to conditions in the Pervasive Developmental Disorder category, and it can be used to convey that a child likely displays Autistic Disorder, without specifying the degree of severity of the conditions. Some forms of conditions in the category of Pervasive Developmental Disorder are mild in severity; some are moderate; some are severe. The same is true of any one type of condition, such as autism. When the term 'autism spectrum' is used, it generally conveys that the condition is not severe, but does display characteristics that may warrant the diagnosis.
Autistic Spectrum disorders are marked by their difficulty in communication/socialization in areas other than the literal meaning of words. It is a spectrum, not an actual disorder itself. You may have heard of PDD-Pervasive Developmental Disorder or Aspergers Syndrome --- these would be "Autism Spectrum". I am sure the doc will explain better, but I hope this helps for the moment. See if you can access this information, it may help? http://pediatricneurology.com/autism.htm -- Good Luck!