I've had a couple people ask me about my new ADHD diagnosis, so I thought I'd share a bit more about ADHD. The following is based on my understanding of talking to my doctors and my reading, but I'm not going to cite any sources. If you have anything to add or correct just chime in in the comments.
Currently, the official diagnosis is ADHD, which stands for Attention Deficit Hyperactive Disorder, whether or not you are hyperactive. ADHD is a bit of a misnomer. It is not that you have a deficit of attention but that you have reduced control over your attention. Everyone is constantly bombarded with varying stimuli -- sights, sounds, smells, thoughts -- all day long. In a normal brain, the frontal lobes perform a series of functions called executive functions which control and regulate the other actions of your brain. They help to determine which stimuli are important to attend and respond to, and also are involved in being able to monitor your behavior, and being able to make and execute plans, among many other things. In a brain with ADHD, the executive functions are impaired to one degree or another. This results in the classic ADHD "attention deficit." The individual can't easily screen out extraneous stimuli and so tend to get distracted. However, they can also become "hyperfocused" and become so focused on one stimulus, such as the computer or a video game, that they can't easily switch their attention to other stimuli in the environment that they do need to attend to, such as the toddler over there dumping out the contents of the pantry.
One of my doctor's described the executive functions as the conductor of the orchestra of the brain, they coordinate all of the other brain functions so that things work smoothly. Without an effective conductor, the other elements of the orchestra don't come together to function as a whole, and the result is chaos. Stimulant medications work for ADHD because they "wake up" the conductor, and the individual then becomes better to direct their attention, to plan (short and long term), to control impulsivity, and much more.
In my case, I really hadn't expected to find out I had ADHD. I had recurring bouts of depression, and once I had children that become bouts of depression and struggles with anger management. My therapist initially though that perhaps I was bipolar, but we did extensive testing and the results were quite clear that I had ADHD, inattentive type. I started medication for this, and the change has been drastic. Before the medication, I spent much of the day flustered, frustrated and overwhelmed. Once I was so flustered, the smallest thing, such as simple noises from the children or the pets, could set me off and I would explode. Housework was usually undone, and instead of working on it, I'd look at what needed to be done, feel completely overwhelmed, and go read a book or surf the 'net. On the medication, I get overwhelmed and flustered much less often. I can look at a busy, overwhelming or difficult situation and figure out how to handle it, instead of getting emotional. I am better able to switch my attention away from things I'm engaged in, even absorbing things like the computer, to take care of other things, without getting angry or frustrated. I can interrupt a task and then remember to come back to it. The difference the medication has made has be phenomenal. I still have done a lot of work in building good habits of thought and behavior, and I still have plenty of more work to do, but I feel like I'm able to do it now.