PHP image pop up

so i’ve been looking around the net and there are some sites that have a link or thumbnail image (not neccessarily wordpress sites) and when you click the image or link it opens an html or php popup that loads the image, the unique thing about it is that it’s not really an actual window of the browser, its specific to the window that is already open, it also has a slick little mac-esc loading graphic before each image, does anyone here know what i’m talking about and can you help me setup how to do it? thanks