... a brain disorder that typically affects a person's ability to communicate, form relationships with others, and respond appropriately to the environment. Some people with autism are relatively high-functioning, with speech and intelligence intact. Others are mentally retarded, mute, or have serious language delays. For some, autism makes them seem closed off and shut down; others seem locked into repetitive behaviors and rigid patterns of thinking.
Although people with autism do not have exactly the same symptoms and deficits, they tend to share certain social, communication, motor, and sensory problems that affect their behavior in predictable ways.
The National Institute of Mental Health has an excellent article describing Autism in detail.
How to Spot It
Autism can be diagnosed as early as 18 months of age with an 83% accuracy. It is a myth that children will "...just grow out of it." There is no research to indicate that this is true- with children that receive no form of intensive therapy. If your health care provider suggests that you wait and see if your child just "...grows out of it..." , the symptoms that you are observing and find troubling, GET ANOTHER HEALTH CARE ADVISOR.
The window of opportunity for improvement, progress or "recovery" is most open in these early years between 18 months and 5 or 6 years of age. This does not mean that older children cannot improve. Just that, the younger your child is when you recognize their problem and begin treating it, the most progress is possible.
The CHAT test (Checklist for Autism in Toddlers) is an excellent tool for both Parents and Health Professionals to use in diagnosing Autism in toddlers and has been found to be invaluable in doing so.
What Causes It
Nobody knows for sure. Children with Autism are described as being on "The Spectrum" (Autism Spectrum Disorders). This refers to how Autism and other conditions are viewed as being related in symptoms but with varying degrees and shades of severity. There are other conditions like Asperger's Syndrome, Pervasive Development Disorder (PDD), Fragile X Syndrome, Attention Deficit Hyperactivity Disorder (ADHD), etc.- that exhibit similar symptoms in some cases.
There is research to suggest that in some cases there is a genetic component that is triggered that leads to Autism. There is also research that indicates that Autism is due to environmental influences such as vaccinations. In the future, it may be found that both reasons (and others) are true for different children.