"What is autism exactly?"

I am continuing to try to learn as much as I can. There are a lot of books out there about autism (almost too many to choose from!) but I picked this book to read next solely because I was so impressed with the care and respect the author showed in his definition of autism:

The word autism is derived from the Greek word autos or "self," as in autonomous. It has been used to describe individuals who appear to be self-contained or who exist in their own little world, an inner realm seemingly set apart from others. These individuals have been clinically characterized as intentionally withdrawn and lacking in social reciprocity due to their communication difficulties or seeming disregard for social norms, as demonstrated through repetitive actions such as repeated hand flapping or infinitely spinning the wheel of a toy truck instead of rolling the truck along on all four tires.
From a physiological perspective, autism is a common neurological anomaly that may preclude the body from properly receiving signals transmitted by the brain, resulting in misfires and disconnects. Thus, people with autism may be unable to speak (or to speak reliably), to move as they would wish, or carry themselves with grace and complete agility. You've experienced autistic-like symptoms if you've ever transposed or stuttered your words unintentionally, or if you've awakened in the middle of the night to discover your arm is "asleep" from the elbow down and cannot be willed by your brain to move of its own accord.
Autism is a unique and different way of being, a natural variation of the human experience. Those who are autistic are often inherently gentle and exquisitely sensitive. They may perceive the world through a multifaceted prism more complicated and interesting than the view of those who are considered "typical." The autistic experience brings many gifts to appreciate and challenges to master...as one attempt to assimilate with the world at large. 

                                                                                   -William Stillman, The Autism Answer Book