Vi (officially pronounced `vee-eye'/unofficially pronounced `six' because of the feeling one gets when using vi that it may be the text editor of the antichrist) is a display oriented interactive text editor.
Vi[+] makes one major philosophical deviation from every other text editor I have come in contact with. The basic idea is that your hands don't have any business straying from the home row keys. This can be an advantage for the touch typist, but the guy who needs to see the letter on the key before pushing it down tends to be less enamored with this characteristic. Since your fingers can only reach about fifty (50) keys without moving your hands, and since vi has in excess of 100 commands, something drastic must be done in order to designate all the functions a decent text editor must have. Rather than relying on extra keys on your keyboard that seem a little too far away or special key combinations that involve keys that your keyboard may or may not have, vi simply assigns a couple functions to the keys in reach.
Vi operates in two modes[+] (insert and command) in order to determine which function should be performed when a key is pressed. This two mode novelty, in my opinion, is what causes some to confuse vi with the devil himself and causes others to place vi equal with God. Although I find it hard to justify worshiping a text editor, I can (after much effort) appreciate the utility of vi.
Why vi? Vi is the default editor for Unix. It is possible to use other editors, but if you learn vi you can be confident that it will be on any Unix machine you use. However, the same level of confidence with another editor may be shortlived. It would be to your advantage to learn at least the basics of vi. For those who use a text editor on a daily basis, particularly for programming, vi will become a joy to use after a few months of friendship building.
As with any friendship, an emphasis must be placed on quality time, not just quantity. Those who have little use for a text editor may be satisfied with a cold professional relationship with conversations limited to a few basic commands. The rest of us would certainly benefit from a little quality time with vi. By quality time, I don't mean merely having the same conversations over and over (repeating commands you already know). I don't mean just reading about what makes vi tick. Although these are important activities, I mean telling vi things you've never told it before and observing its response. Don't discuss important issues with vi until you're pretty sure you know how it will react. Make sure you make a backup copy of the file you experiment with.