Networks when visualized are usually represented in a circle. Many networks (circles) overlayed on top of each other (as the internet is many smaller networks) when represented visually begin to look like a cloud.
For the last 15 years I've been in the ISP industry, when you draw a network diagram, you draw your own network, then draw links to an actual drawing of a cloud that represents the rest of the Internet.
Somewhere in the last few years, someone took that cloud in a drawing to mean where stuff goes to be stored on the Internet.
reply to axiomatic CCNP myself. We'll just have to agree to disagree on this one then. I had this taught to me in multiple cisco classes. Not saying they're right but that sure is a lot of credible people wrong about it then.