Here is another post from "Autism Discussion Page". It's a nice explanation of different behaviors shown by kids with autism. -
Differences between Tics, Compulsive Behavior, and Self Stimulation
Are your child’s repetitive behaviors tics, compulsive behaviors, or self stimulation? The repetitive behaviors in all three conditions serve different functions. Tics are a neurologically driven, often involuntary. They can be very jerky, not a smooth rhythmic pattern, and often involuntary. They usually involve motor musclegroups (eye blinking, facial twitching, etc.) or vocal noises, sniffing, throat clearing, or snorting type patterns. Usually the child has multiple motor patterns, and at least one vocal tic. They seem to serve no functional purpose for the child. The child may feel the “urge” coming on, but have very difficult time stopping them. The child do not enjoy the tics, and often feels frustrated in not being able to control them.
Are your child’s repetitive behaviors tics, compulsive behaviors, or self stimulation? The repetitive behaviors in all three conditions serve different functions. Tics are a neurologically driven, often involuntary. They can be very jerky, not a smooth rhythmic pattern, and often involuntary. They usually involve motor musclegroups (eye blinking, facial twitching, etc.) or vocal noises, sniffing, throat clearing, or snorting type patterns. Usually the child has multiple motor patterns, and at least one vocal tic. They seem to serve no functional purpose for the child. The child may feel the “urge” coming on, but have very difficult time stopping them. The child do not enjoy the tics, and often feels frustrated in not being able to control them.
A true “compulsive behavior” (as compared to a fixation) is driven by anxiety and often result from “obsessive thoughts”. The person feels driven to do it because of anxiety and will become more anxious if you block it's occurrence. They are aware of and have some control over the behavior, but feel “compelled” to do it, and feel very anxious if they don't do it. They often do not feel good, the child is driven to do it, because they feel anxious if they don’t. The disorder can consist of strong repetitive obsessive thought patterns (intense worries, exaggerated fears, etc.), compulsive behavior patterns (washing hands over and over, repeatedly opening and shutting doors, compulsive orderliness, etc.), or both.
Self stimulation is a more voluntary, controlled sensory seeking to calm and organize the nervous system. These repetitive behavior is used to sooth and calm the nervous system when over-aroused, alert the nervous system when under-aroused, and block out unwanted stimulation when overwhelmed. They usually consist of repetitive “rhythmic” movement, auditory, or visual patterns. Self stimulation is usually a voluntary pattern the child uses to help regulate their nervous system. Usually if the repetitive pattern, is rhythmic in nature, and feels good, it is not tics or OCD, but self stimulatory.