It is a very real condition, though it's also grossly over-diagnosed. I have ADHD, and trust me, it's about SOO much more than not being able to pay attention. We're an over-stimulated society, so it's amazing when anyone is able to pay attention. I may go from being unable to focus on anything to only being able to focus on one thing for days at a time, have petit mal seizures every now and then, and can't speak properly at other times. Most notably, it's gotten worse since childhood -- I was diagnosed at 14, I'm 28 now and it's more noticeable than ever.
I did write an article recently (not on here, so I won't link it) on some interesting studies about brain structure differences in ADHD. Apparently, most of the study subjects had differences in their frontal cortex, basal ganglia, memory centers, and tissues around the parts of the brain that control concentration. There were a few other differences noted, but the point is that they were able to see a notable difference that wasn't a simple chemical imbalance. Note, too, that all of the people used in the studies were diagnosed by specialists, no self-diagnoses or labeling from pediatricians, school nurses and such like that. It's some pretty fascinating reading if you like science and want to understand the condition a bit better :)