Vim is "VI iMproved" and it's one of the original text editors. It's one of the few from that time period that's still used today, and very actively.

It's also one of the things that has been in "Holy Wars" in Hacker culture. The one with Vi / Vim is "Vi / Vim VS EMACS" as EMACS is huge, and has a lot of features, where Vim is a full screen text editor that doesn't use a GUI, and uses commands to save text, move the cursor, and in general, write something or change text. In Vim you start in Command mode, and if you want to try, you have to hit "i" which then inputs text, and then if you want to save, you hit Esc, then ":w" will write the text, and ":wq" will write it to file and quit the editor. There is a lot more obviously, but that's a basic run down.