ADHD is known as Attention Deficit Hyperactivity Disorder.
A variation is ADD which is the same without the hyperactivity.
ADHD is a disorder that is seen primarily in boys, in which they seem not to be able to control their actions, find themselves often unable to sit still, act without thinking of the consequences and unable to follow instructions or complete basic multistep tasks.
ADHD is caused in many ways but in most cases it is either due to complications during pregnancy or birth, a head injury, exposure to a toxic environment, a traumatic event, genetics or an accompanying disorder.
It is classified as a learning disorder but it is definitely more of a neurological disorder that requires some sort of help beyond tutoring and other intense learning sessions.
ADHD's broad definition gives plenty of room to critics who say that this is a disorder brought about by the new society that we live in.
A time with both parents are not involved in the child's learning process, set into a structured schedule from birth and given all sorts of high energy, attractive and very entertaining toys and tools at a very young age.
All these things combined leave little room for peace and quiet so that the child learns the benefits of self control and moderation.
The ADHD children are just over stimulated by too much for too long at too young of an age.
By the time they hit school and it's quiet and mundane tasks, they will not be able to concentrate.
They never learned how.
The ADHD definition most commonly used is that it's a disorder that effects the ability to concentrate and focus for any length of time.