"vi" (pronounced "vee eye") is a text editor with a deceptively simple appearance that belies its great power and efficiency. New users soon realize that there is far more to this little program than meets the eye.
vi, or one of its clones, is found in almost every version of Linux and Unix, and, in fact, it is the only editor that is available in virtually every Unix installation. Moreover, clones of vi are available for nearly every other operating system in use today, including Microsoft Windows. This makes it one of the most widely available programs in the world!
This near-ubiquity plus its power and efficiency (at least for experienced users) makes vi one of the most important programs for students of Linux to learn. And a command of vi helps differentiate a beginner from a true power user of Linux or Unix.