They forget or lose things like homework, books, or supplies. Some kids just make a lot of careless mistakes on school work. It's not because they don't care, but because many times they get distracted, or they get very impatient and rush through things. It just means learning how to do the right things that help you do well. Your teachers, parents, or a counselor can give you some good ideas on that.
Doctors know that it is genetic and runs in families.
Most people are born with it. If you have ADD, chances are that at least one of your parents, brothers, sisters, cousins, aunts, uncles, or other close relatives has ADD too. They may have too little of some brain chemicals, called neurotransmitters, in the parts of the brain that control attention and impulses. This is a biological thing, it is not an illness or a disease. People with ADD are just as healthy as anybody else.