What is the difference between a wallpaper and a theme?

Wallpaper (also known as a desktop background) is a usually static image placed on the desktop area of many personal computers that use a graphical interface. Screen savers are programs that display moving / rotating images or animations after a certain amount of time with no user input has occurred.

so really what I'm saying is if you want to look at it right away, its a wallpaper, if you see it say, 10 minuets later and its moving, its a screen saver.