vi is a Unix/Linux text editor used for creating and modifying a range of different types of files. Use of vi can be cumbersome at first and requires knowledge of at least 15 or so special commands in order to be useful.
At linux prompt type "vimtutor" (without the quotes) to start a built in tutorial
See vi on Wikipedia for a list of commands.
Here is a link to a vi reference card that lists all the commands.