Guitar Tablature file
Tablature is musical notation that depicts instrument fingering rather than the standard notation symbology seen on scores and sheet music.
A TAB file is essentially a text file that contains song lyrics and guitar tabs, only differentiated from a TXT file by its content and file extension.
Learn more about TAB files:
See many tabs at MySongBook.com.
Ask a question about TAB files at IT Knowledge Exchange.
Wikipedia has more information about tablature.