Linux debuted in 1991, and since that time it a gained so much momentum that it's fair to say that it runs most of the internet with one flavor or another. Linux is unique in that it's open-source and can be freely modified and redistributed under a different name. This means you can't charge for the operating system but you can charge for add-ons or service and support. This is how companies like RedHat became profitable while providing their flavor of Linux.
To interface with a Linux OS, you use a shell, which gives you access to the operating systems' services. This typically comes in two visual forms of a graphical user interface (GUI) or a terminal with a command prompt. GUI provides a nice point-and-click interface and they are very useful when operating locally. However, GUIs tend to be a waste of space and resources for cloud services, so it's good to know how to use the command line. Plus using a command line can actually be much quicker once you get the hang of it.
While this section may include a few useful GUI tips from time to time it will mostly be updated with command-line documentation.