What is "base" exactly?
 

I am new to cycling and I have read alot about about the best ways to improve. Lots of sources suggest building up a large amount of "base" miles at the beginning, which I am trying to do right now.

If I understand it right, this means riding at lower levels of intensity in order to increase your body's efficiency at aerobic activities, and to get your body used to long distances to avoid injury.

I have searched alot of threads and read all the sources I can find, but I still have a question:

Half the things I read suggest riding in zone 2 (60-70% max HR), and half say zone 3 (70-80% max HR).

Which one of these is more commonly referred to as "base" riding, and which would be more useful to me?

Base means you've ridden a bunch of miles before you start paying attention to zones. Go out and have fun!

^+1
It doesn't really matter, just don't beat up on yourself. Keep it fun. I do a mix of zones year 'round. It's not necessary to keep it down, in fact it works better to do some hills, just don't knock yourself out on them. A minimal base is usually 1000 miles, more is better. I try to get in a couple thousand before I do anything serious. So I'll start base training in October and do my first intervals in February. This year it's too late to start a structured plan unless you live in Oz. So just ride. do what you want to do, and see how it comes out.

Haha--thanks! I originally started cycling as cross training, and now I think I'm hooked!

Whatever effort level allows you to ride consistently for longer periods e.g. 3+ hrs. If you ride 3 hrs and you can't or don't feel like riding long the next day then you went too hard. Generally, a comfortable pace that doesn't leave you wiped out after 3-4 hrs.