As you play guitar, you are going to want to learn all of your favorite songs. Now you can go and buy the original sheet music and decipher it and learn to play it, or you can look up the tabs that have been made on it. Tabs are the easiest to understand way of reading music.
While sheet music shows you what notes to play, tabs show you what string and fret to play. This way you don't have to go through the trouble of figuring out what each note is on a guitar.
Be warned, however, tabs are very often inaccurate. Tabs are usually created by a guitarist who beleives they have figured out how the song is played. Unless you have the original sheet music translated into tab, you cannot be sure that it is correct.
Reading guitar tabs is an easy thing to learn. An example of guitar tab is pictured below.
When you look at this it might seem like a lot of nothing. The numbers represent the frets and each line represents a string. In most tabs the high E will be on top with the low E on the bottom. The high E is always represented with a lowercase "e" and low E is represented with an uppercase "E".
As you move from left to right on a tab sheet, time is passing. Meaning that in the tab above, you play the 7th fret on the D string before the 5th fret on the G string because it appears further left.
When two numbers appear in a vertical line such as the 7th fret (e string) and the 6th fret (D string), you play both notes at the same time.
People who create tabs often have their own way of representing things such as slides, hammer-ons, and hammer-offs. They will usually put a key at the top of the tab to help you understand it bettter. At the end of the tab above you will see a '/'. In this tab it represents a slide. Since it is followed by an 8, you would end the slide on the eigth fret.