Floating Flash?

Hi guys
I’ll do my best to try and explain this.

Have you ever been on an HTML site and in a corner of the site there is a flash ad or a banner that when you mouse over, it takes over the entire page?

I always thought that with flash your always constrained to the size of the movie but it seems that there is a way to make your movie “float” over HTML when “on mouse over”

If I could get some help on this terminology to find a tutorial or if you could provide some help it would be greatly appreciated.

Thanks