VI is the visual interface for EX¹. The close link between these two programs is more useful that one thinks, because if the bad gets worse, VI will switch into EX-mode when your terminal settings are broken so badly only EX can still work. This means that if your UNIX terminal breaks down because of a bad configuration, you will be able to kludge the configuration files to get the usual display facilities back.

VI - a definition

Vi (visual) is a display oriented interactive text editor. When using vi the screen of your terminal acts as a window into the file which you are editing. Changes which you make to the file are reflected in
what you see.

William Joy in An Introduction to Display Editing with Vi

William Joy is the author of VI

Why learn to use VI?

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  Like many powerful tools, VI is somewhat difficult to learn if you want to get all the power out of it. It is quite easy to learn rapidly how to type text, save it or make simple editing tasks. But if you have complex editing to do that e.g. you could not achieve with Notepad (if you are using Windows), VI can be the solution to your problems.
  

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  VI can be configured to suit your needs. You like indentation in your program files? No problem, you can set how much each block has to be indented and you can even indent a group of lines with one command only! You can also define macro's to complete your typing (that comes in very handy when you have to close all these HTML-tags)
  

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  VI has a very complete set of possibilities for search and replace actions (look at a good tutorial for that and other aspects of VI)
  

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  VI is present on nearly every operating system/machine, and doesn't need long customization before you can get to work with it: look at the list of free domain/shareware programs.