Well, I haven't had a child with it, although I have looked after children with it.

Essentially, autism is a disorder whereby children (and adults) are unable to read or react to the normal social cues that are present in communication between individuals. They are often withdrawn or inappropriate in their communications for this reason. It is a "spectrum" disorder in that it affects different children differently, where some might be midly autistic and have some interpersonal problems but overall can function in society, to children whose inability to communicate is significant enough that it impairs their normal social and intellectual development, to those who are so affected by the disorder that they become withdrawn from the outside world.