looking for a particular software

Ok! So you know how the new keynote has that alpha thing... that you can leave all out except for the picture. For instance if i have a picture with a palm tree i can take everything away(sky, sun, etc.) and leave just a palm tree.
Now, is there a soft where i can do the same thing with video? Where i can take everything out and just leave for instance a cat walking or i don't know, something like that. you get my point? If you do, than please help