The first time I built and installed the sample HelloWorld app I was annoyed to find that it didn't include a way to close itself. It has only a "Push Me" button which opens a page to ask for your name so it can greet you with it.
As it says carefully hidden away on this page it's quite simple to close any app, or to switch between them. That page describes "swiping" with the mouse from bottom to top of screen. This has the effect of minimizing the application into a region showing it and all other active applications (if there are any).
The app shown in the centre has a red X widget to let you close it, but you can actually close any of the applications shown by dragging it up to the top of the screen, sort of like a "toss it" gesture. Feels nice... Play a bit and see what else you can do by dragging and swiping around.