Whats the difference between Wallpapers & Themes?

What is the difference between a Wallpaper and a Theme? Is a wall paper more like a screen saver shot and a theme more like an "operating system?" For instance, will a theme change options like how many screens I have access to and what my shortcuts or widgets look like? Is a theme just a program that I can choose different backgrounds w/?
I'm sorry but I'm so confused

A wall paper is a graphic or picture that you set on your home screen.
Any and all App Icons will be displayed over this.  Think of it like its
name sake.. Wall Paper is used to decorate a room in a colorful way
with a unique color or pattern.

Now a Theme is a bit more involved.

A new Theme is usually a custom enhancement that you add to you phone
that may add a picture or graphic like a wall paper, but also can change
your environment as well.  App Icons can change as well as other features
(Your phone's clock for example)

Can't tell if you're trolling by bringing up a thread from a year ago, but it really can't be explained any easier. 

Just like with your regular computer, a wallpaper is the pic that shows up on your desktop.

The theme changes the overall look and feel of the computer/cellphone like the colors and the fonts of everything.