Originating from the renaissance period (14th to 17th Century), tablature has been used for many fretted instruments such as guitar, lute, mandolin, banjo, and vihuela.  To this day it is still popular and its use for guitar is so widespread that it can be found on its own or with standard notation in many books, magazines, computer programs, and on the Internet.

Guitar TAB is made up of 6 Lines that represent the 6 strings of a standard guitar.  The numbers that appear on top of the lines indicate fret position.  

The top line of TAB is the 1st/E String (Thin)

The bottom line of TAB is the 6th/E String (Thick)

The above example indicates the following:

1st String - Open

3rd String - 3rd Fret

4th String - 12th Fret

6th String - 6th Fret

When starting out with TAB, it's important to always read it the same way.  Ex.  figure out the string first and then the fret or vice versa.  Try not to go back and forth until you can read TAB very quickly, otherwise you'll find that you get mixed up a lot.

Although TAB and standard notation are both valuable, TAB allows beginners to read music immediately.