Guitar tablature is a form of written music in which the fret and string to place your finger on is shown in order of how it appears on the song. Sometimes, various methods are used to denote timing, and often effects such as "slides" or "bends" are notated as well. This is the only way to learn many popular songs (unless one transcribes it straight from audio) and is an easy, free way to learn songs on guitar with little or no previous musical knowledge.
There are many sources of guitar tablature online and in most cases these are in the form of html documents, displaying the written music using text characters. However there are also guitar tablature software which allow the user to write, edit, listen and print out tablature. These programs are infact becoming so powerful that many of them support multiple tracks, making it possible to tabulate all the instruments in a band in one tablature file.