Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurological brain disorder that manifests as a persistent pattern of inattention and/or hyperactivity/impulsivity that is more frequent and severe than is typically observed in individuals at a comparable level of development. ADHD begins in childhood, and as has only recently been understood, can persist into adulthood as well. While some children outgrow ADHD, about 60% continue to have symptoms into adulthood. The disorder manifests differently in adults as hyperactivity tends to diminish.

There's more to it than that though; that's just a brief description, also it's just what you've been diagnosed with, it in no way defines who you are as a person - everyone is different to everyone else anyway.