I would like to gain some knowledge about hops and have an idea in what all those varieties differ. Is it possible to define a few categories for hops (for example based on region of origin), so to choose one variety from each group and start knowing the differences between different categories?
here is a rudimentary takeoff on the flavor wheels you can find.
this one is for hop aromas/flavors:
Now that this question is "reopened" I have found a couple of very good resources for this:
Hop Union has an aroma finder: http://www.hopunion.com/aroma-wheel/ The way this is set up is that each aroma is split into 3 groups and each group is alphabetical (so where the alpha order "resets" is the start of the next group) and each hop has up to 3 aromas and the higher the group the more dominant the aroma is for that hop. Ex. Citra is in group 2 for Fruity and group 1 for Citrus so Citrus is a more dominant trait.
Another good resource is the Barth Haas Flavor Wheel: http://www.barthhaasgroup.com/wheel/ They also have this in a variety portfolio PDF version: http://www.barthhaasgroup.com/images/pdfs/hop-varieties/13-10899_uebersicht_Rohhopfen-EN_FH.pdf
(I know links are generally frowned upon since they could go away but I'm not sure how to embed this data since its a bit more complicated).
I'm late to the game here, but for anyone else who lands here, try this: http://www.bertusbrewery.com/2013/03/dry-hopped-bud-light.html