After Effects is a compositor and a motion graphics application, not an NLE. Premiere Pro is the Adobe NLE.
Start here to learn about After Effects:
I'd recommend against comparing After Effects to Nuke, though. Nuke is a compositor for single shots and very little else. It is very good at that one thing, but it is narrowly focused. After Effects is a broader application, with---as I said above---motion graphics capabilities, et cetera.