Very quicky, I can say that classical theory and most other folkier theory to a certain extent states that I or your Tonic is the home base. You start from there, get as far away as you can, creating dissonance and tension (jazz taking it to its logical extreme), then find your way back home through resolution. Through tension and resolve you set up little journeys for the ear, intriguing it then giving it what it wants. I tend to think of it sometimes as a sexual experience, with the grinding tension of the act that feels good, but then the payoff is the release/resolution at the end. kinda weird, I know, but hey, I am convinced thats why we like music (biologically speaking).
Any way, some chords really want to resolve to I, They are called cadences and have names like perfect and imperfect. The most common are V-I (perfect, I think) and IV-I (imperfect, I think) IV-I is your typical Ahh-Mennnn ending at the end of hims. and some have little mini resolutions to each other, but generally leading to that big ol Gspot tornado at the end leading to I. VII has so much tension in it that if you leave it hanging, with out resolving to I, the listener will not like it and will feel unfulfilled...Kinda like, well, you know.
Anyway, other people can explain this much better than I can and there are some awesome maps you can use that I found off of this site.
There is a simple map:
Then the macdaddy map...
Information is not knowledge. Knowledge is not wisdom. Wisdom is not beauty. Beauty is not love. Love is not music. Music is the best.- the girl from the bus