It is said in this document that the

It is said in this document that the "hyperactivity is a state of excessive muscular activity", which doesn't include an excessive brain activity, but doesn't mean that there is no link with the brain.
There is a non-exhaustive list of typical behaviours such as constant activity and moving, impulsiveness, inability to concentrate, aggressiveness, fidgeting, excessive talking...
I noticed that in this document, they talk about the social problems a hyperactive child can face : even if it's more a problem for parents and schools than for the child himself, we can't ignore that for many hyperactive children, the ADHD leads to misunderstandings by the others. That is why difficulties at school in their work as well as difficulties in social relationships with the other pupils or the adults bring unhappiness, depression or at the opposite, agressivity.