Tab is a simple way for guitarists to read music solely for guitar, rather than using the traditional 5 lined method.  The strings are drawn as lines, and the fret which you play is shown as a number.  Look at the tab for the "mission impossible" theme tune, a beginners favorite.

E--------------------------------------------------------------------------------------------| B--------------------------------------------------1--------1--------1---------------------| G-----------------------------------------------------2--------2--------2------------------| D-----------0----------------------------0-----------------2--------1--------0---------0-| A-0--0--3------0--0--------0--0--3-----0--0--------------------------------3---------| E--------------------3--4--------------------3--4------------------------------------------|

As you can see, the bottom string is at the top of the piece, and the top at the bottom.  It is important to remember this.