I don't really understand the % grade thing, and don't have any way of telling, so the only ones I know are ones that have signs when you are going down them that say "7% grade, next 12 miles". Those signs aren't my favorite, especially when I know I'll have to come BACK UP! I think 5-6% climbs are pretty intense. We have a mountain here (Mt. Lemmon) that is a 5-6% grade for 23 miles. You're a pretty good climber if you can hit mile marker 11. I've yet to try it and don't really ever want to.

As for what is a hill, I know it when I see it! Pretty much anything I shift down to my third chainring for qualifies. Although we also have 3-4% grades for 10+ miles, and I don't ride those in my lowest ring, but they are hills, and usually called climbs.