In the 1970’s through 1990’s telecommunications companies needed a simple way to draw a complete network.  It was easy to draw several branch offices and a company headquarters in a circle, label them site A, site B etc. or by city and then draw a cloud in the center of the locations.  Then you only needed to draw arrows showing the direction of communications, which was largely at the time from the remote locations to the headquarters and back.  Today “the cloud” can be much more complex because not all of the locations are company locations and more organizations have multiple type of communications from multiple providers.  At is heart, the cloud means “not here” and implies some level of outsourcing as someone else “not here” is providing a data center environment with computing, storage, network connections, security and and support.  Old habits die hard so we we still call it “the cloud” and draw it the same way. For a good description with drawings this link  will take you to a more detailed article with pictures written by Matt Weinberger of Business Insider titled Why ‘cloud computing’ is called ‘cloud computing’.