When do the symptoms appear?
ADHD behaviors usually become apparent when a child is between 3 and 6 years old. Parents often notice the hyperactive/impulsive behaviors first, when a child is a toddler. Signs of inattention may not be picked up until children go to school, when they are expected to focus for longer periods of time than they may be at home.
But isn't that stuff normal?
There is obviously no child who doesn't display these behaviors occasionally, so the rule of thumb is that kids with ADHD display them three times more frequently than their peers do. Each child's behavior must be considered relative to others at her developmental level. And it's important to note that a child should be compared to others of her age -- not in her grade in school -- because a typical classroom has a spread in ages that may be significant developmentally.
One reason ADHD can be difficult to understand, and diagnose, is that it's what we call a "dimensional" disorder, which is to say that the behaviors involved are present in all children; the difference in a child with ADHD is one of degree. Think of them as the extreme end of a spectrum.
For instance, any child may want to borrow or use other kids' toys, but a child with ADHD constantly grabs toys from other children. Any child calls out or interrupts every once in a while, but a child with ADHD does it so often it's disruptive or annoying. Any child will occasionally fall down or have accidents, but a child with ADHD will have frequent accidents and may end up in the emergency room. Any child might occasionally dart away from a parent on impulse; a child with ADHD is so impulsive, parents need a leash to keep him from running into the street.
Another way to think of the difference is that although all kids may exhibit these behaviors, highly energized, active children without ADHD can usually focus when it is necessary to accomplish a goal. Kids with ADHD can't.