What is ADHD exactly?

I always hear people comparing ADHD to diabetes or high blood pressure when they say things like, "would you take insulin if you had diabetes or BP meds if you had a cardiovascular problem?".

It's my understanding that ADHD is a brain disorder that seems to affect the motor, emotional, 'executive function', areas of the brain. It seems it affects every function from sleep to how they communicate with others to angry emotional outbursts! 

It also seems the denial people with ADHD have about their disorder affecting others is comparable to a person with a mental illness not wanting to take their meds. because they strongly feel that they are not Manic or ill. (I read this in 'Is it You, Me or Adult ADD' under the section of 'denial')

Am I wrong about this?

It seems more like a mental illness than a physical illness, though I understand it's basis is a brain development issue--right from birth.

I'm also wondering what symptoms can be controlled/managed, which symptoms will just go away completely with which treatments, and which symptoms are just something we will have to live/put up with for life? :/

I know clearly that if a person with Manic Depression takes their meds they will seem pretty even in their emotions, they will be able to live almost "normally" in their every day lives. Take the medicine away and it's a predictable outcome: the mania will start to show in all the eccentric ways they normally do in that specific person. They eventually cannot function, maintain themselves, and it can get to the point where they are a harm to themselves or others because they are no longer 'living' in this world... they're living in a manic world. Therefore hospitalization is necessary at that point to get the person back from that state and into our "realm" again.

With ADHD it's not really that predictable to me.

I was wondering if anyone else has got it figured out where they've found a pattern in ADHD. Like, the perfect formula to get the person acting somewhat "normal" and fully functioning. 

So, is it physical (brain development) that thereby affects the chemical balance in the brain, thereby affecting how the person acts emotionally and physically towards others?

I'm a bit confused.... Help!