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:
And then there are some little details that you'll need to know to fully understand how to read guitar tablature. Be aware that some of these symbols may vary, depending on who created the tablature.
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.