Do you REALLY want to know? It is Gaussian Blur. Hard to explain. But it is the tool that makes collages great, is needed badly for manipulating masks, alpha channels and will a lot of times be used in combinations with special selections. Like in creating depth of field. And it is great for creating light effects and text stuff. I have fallen in love with blurred layers in different layers modes. Like this pic, which needed more of a movie-look (everybody sends me jpgs these days). Just blurred layers and a bit of color tweaking. For this and many other things its Gaussian Blur.