What is Attention Deficit Disorder on adults and how is it treated?
It doesn't sound like your just looking for some general info about the psychological disorder of ADHD. The thing you should maybe start out with is understanding that there are two different sub-types which will have different effects. 

Rather than explain everything, I would point you to a couple links that should answer your questions: 

For a basic definition you can look at the CDC page on ADHD.

Here is some basic info that should get you started on understanding the disorder. 

wikipedia should fill in anything else that you want to know, if you have a more specific questions, feel free to ask.