Forgetful, twitchy, energetic, flighty, impulsive. 

Kids with ADHD have difficulty paying attention--this can range from being overly hyper in class to simply staring off into space.  They often can't sit still but when they are doing something that they really enjoy can be completely involved in that task to the point of nearly seeming obsessed with it.  However, given a task that they find boring they will never be able to attend to it.  You may find yourself saying something like, "I've just asked you do one thing.  Pay attention to what you are doing and finish that one task."  Something  simple like teeth-brushing becomes an almost impossible task.

ADHD behaviours can seem disruptive but even though the child may seem disobedient, they often are not trying to be bad.