Speech delay and/or dysfunction are considered to be a primary symptom of autistic spectrum disorders. The issue is usually lack of speech due to an apparent inability to understand speech as a vehicle for symbolic communication.
This does not mean that a person on the spectrum cannot also be affected by another common speech disorder. In fact, they are far more common in people with PDDs than they are in the general population. Speech disorders come in many flavors. The descriptive terms you will see most often are:
Aphasia: difficulty in using oral language (expressive aphasia) or aural language (receptive aphasia). Can be characterized by pauses in speech, or seeming to be "deaf" at times. Aphasia is usually associated with brain injury.
Stuttering: getting "stuck" on a sound, usually a single letter or syllable at the beginning of a word, and repeating it.
Cluttering: getting "stuck" on a complex sound, such as a word or phrase, and repeating it. Also characterized by a fast, fluctuating rate of speech.
Verbal dyspraxia: disordered speech due to underlying problems with muscle control. Also called Developmental Dyspraxia of Speech.
Oral Motor Apraxia: lack of speech, or disordered speech, due to underlying problems with muscle control.
Technically, apraxia refers to the absence of speech, while dyspraxia refers to disordered speech. In the U.K. and some other countries, that's how these terms are used. In the U.S. and Canada, however, the terms are used almost interchangeably, leading to much confusion. Be sure to ask what any diagnosis you or your child receives is actually intended to mean.
With a non-verbal child, simply producing sounds that have communicative intent is the initial goal of speech therapy. As children become verbal, oral-motor problems, stuttering, and such may emerge. Speech therapy should be tailored to fit the child and the problem. Stuttering, for example, can often be remedied in weekly group sessions with "homework." Oral-motor apraxia is a much more demanding condition, requiring intense, frequent sessions.
For more on Apraxia and Dyspraxia, look under ASD/PDD - Other Conditions on this website.