There has never been as far as I know of, a concrete medical diagnosis for ADD or ADHD even though drugs are commonly prescribed for the illness however, they are working on a 'real' diagnostic set of tools that will evaluate true ADD. The illness was first labelled as a result of psychiatric groups getting together and making that decision. Some of the drugs given to children for this behaviour have never been intended for children and the long term affects are still not all known. Everyone has a different answer as to why it is now so prevalent and the answers range from vitamin deficiency, poor or no parental control, bad behaviour, allergies, premature births etc.
Dexedrine and Ritalin are among the most well known pharmocological agents. Amphetamines have been around since the 1880s and have been studied for just as long. The FDA approved Ritalin and Dexedrine for ADHD in the 1950s and have been proven effective for decades on people that benefit from them.
Less blood flow to the brain. Less glucose metabolism in the brain. 70% more DAT transporters in the striatum. Lower levels of dopamine throughout the brain (using chemical markers and SPECT scan). Overactive RAS and thalamus. Structural differences in prefrontal cortex. Polymorphism of the DRD4 resulting in the 7 repeat allele. Audio and visual processing diagnostic tests. It took me several months and trips to many specialists (developmental pediatrician, psychiatrist, specialist doctors, etc) for me to be diagnosed when I was a wee one.
There's been more data on the brain differences in the past 4 years than all the rest combined. Since we've mapped the human genome and actually can identify which genes are specific to which behaviour traits, they are beginning to understand exactly which genes and brain differences account for ADHD. I encourage you to simply look up DRD4 in google and read the government webpage that published all the findings of recent research into that receptor, they are now convinced this gene is directly responsible up to 30% for the symptomology of ADHD.
Also, I just had to mention that these medications are highly variable to people. A common side effect of amphetamines and other stimulants like methylphenidate (Ritalin) is increased socialisation, and that's a positive thing. Dexedrine has improved my socialisation ability which allows me to come out of my shell more often. It makes me if anything less violent. I'm more violent when my mind is racing and I'm in a state of agitated depression unmedicated.