There is a good write up here on the different schools:
http://www.alanpeto.com/buddhism/understanding-mahayana-theravada/
Unfortunately, because I have only really been following the Theravada school, I do not know of any Zen Monasteries in the US. I know of a few Theravada Monasteries (and have visited one in San Diego), but you should be able to find some Zen Monasteries in your area; most come up on Google search and will have information about retreats, etc...