If you go to
Zones on the left menu you'll see the expansions, from which you can click on each one to see what zones are in which expansion. Generally speaking, the different expansions are broken up by continent or alternate planes (and the moon for Luclin).
For the most part, it's not quite so important to know which zones belong to which expansions. It's something you'll pick up in time for sure, and you may never learn where EVERY zone is.