I've read that autism and ADHD share several symptoms. It was once thought that they were mutually exclusive, but now it's known that there is a large comorbidity between the two.

Can someone explain, for a layperson, how they differ in terms of their physiological cause and/or mechanism? In other words, obviously there is something wrong with the brain for both ADHD sufferers and autists; how is this "something wrong" different between these two groups?