You can do 2-d animation by the following.
Draw your frames on paper (on a lightbox, so you can see your previous drawing, and tweak the second one slightly when you trace over.) Then photograph each frame, at the exact same angle. You can then, like stopmotion, play them on monkeyjam and see if you like the animation. As it will only be a black and white drawing, you can then import each frame to photoshop. Put the photographed drawing as the back ground, and on a new layer, draw over (using colours,etc) in neat. Do it for very frame, then import them all to monkey jam. Then export as a movie.
Its simple.
Erm, Chocochoc is talking about 2D computer generated animation. I don't think he's asking about classic hand drawn technique.