How to create "2d" animation?

Yes, it is as simple as keyframes (or tweens in Flash)... However... It totally depends on the style of animation you are going for. I've done animation in Flash using keyframes and I've included the link, but, as you can see, animating this way looks very far from professional unless you REALLY put a lot of detail into it. I didn't. But for a two month project, I think it turned out pretty well. Anyway, in order to make something that is truly impressive, I would suggest doing frame-by-frame in Flash with extreme attention to detail and motion. This is just my suggestion, and I have seen tweening animation that is actually very professional looking (usually on Adult Swim), but they are few and far in between.