Many will remember the Rain Man character played by Dustin Hoffman. This character captured in a very general and stereotyped way a few traits people with autism display: social awkwardness, rigid and repetitive behavior, and amazing genius.
But, just what is autism?
Autism is a spectrum disorder, meaning someone diagnosed with autism may have any variation and/or severity of the symptoms of this neurodevelopmental disorder. But the three common symptoms of autism are impaired social interaction, impaired communication, and narrow interests and repetitive behavior. For the person that is not affected personally by autism, it might be presumed that just because someone with autism doesn't speak, that they have nothing to say. Nothing could be further from the truth. I encourage you to view this video made by someone with autism. (It's about 8 minutes, but truly educational)
I'd also like to point you to the Autism Speaks website, which talks very plainly about the autism spectrum and has many resources for families, friends, educators, etc.