Guitar tabs consist of 6 horizontal lines (strings, the bottom line is the thickest string) and they are marked with some numbers. The numbers tell you which fret to hold on that string. A '0' means that you play the string open (unfretted). If there is no number on a string, you simply don't play that string. There is another thing about the position of those numbers - take a look at this image to see the difference:
Hammer On: It means that you hold the 7th fret with your index finger and then hit the 9th fret with your ring finger hard enough to make a sound of the 9th fret. It is usually written as 7h9. Sometimes they just put 7^9.
Pull Off: Imagine the situation after the forementioned hammer on. Now you can pluck the string with your ring finger to make a sound of the 7th fret. This is called a pull off. Usually it's written as 9p7 or 9^7.
Bending a string is written as 7b9. The return of the bent note is marked as 9r7. Some sloppy guitarists use the 7(9) notation.
An ascending slide is marked as 7/9 and a descending slide is marked as 9\7. And sometimes you will find the 7s9 notation.
Vibrato is notated with a ~~~ or v.
A string mute is shown with an x.
The harmonics are usually indicated like <7> - for the harmonics, played at the 7th fret.