Guitar tablatures (or tabulatures, or tabs) are a method of indicating the fingering of guitars for playing particular musical pieces. It was done in writing (for various other instruments as well as the guitar) for centuries, but is now commonly done in electronic form for guitar players to instruct one another in playing songs by exchanging tablatures through the Internet.
Tablature files are generally plain text (which could be in the form of ASCII text files, or embedded in HTML or other document formats) including dashed lines and various letters, numbers, and standard ASCII punctuation characters depicting the strings of the guitar and what the player is expected to do with them at each point in the piece. To line up properly, tablatures generally need to be viewed in fixed-width fonts, since proportionally-spaced fonts will probably mess up the formatting.
Many websites have been set up for the purpose of exchanging tablatures for popular songs, but in some cases the music publishers have reacted with legal threats or actual litigation because the tablature files are viewed as infringing on the music publishing rights for the songs.
Some sites where guitar tabs can be found: