Does everyone have a touch of ADD?

I was with some friends yesterday, and one of them has ADD, they don’t listen very well and always try to change the subject, no matter how important it might be.
I don’t have ADD but I find myself doing those kind of things sometimes. Actually I find pretty much everyone does at some point. So what is the defining line? I don’t know much about ADD, but what makes one person have it and someone else who acts the same way, only slightly less often not have it?
Why do people with ADD need drugs? That has never made sense to me.
Can someone explain this for me?
Thank you!