How to make odd-shaped video animations- IR

I've seen animations that were taken from music videos or movies but not the typical square or rectangular shape but asymmetrical etc. I've even seen an animation shaped like a cloud.

How the heck do I do that? :confused: I know how to make animations but how do I get them to be oddly shaped. Any help would be greatly appreciated.

Most I have seen were produced with flash by simply masking in the shape they want.

You can do basically the same thing on animated gifs in Photoshop/imageready

And how would exactly could I do that, I haven't a clue.