The long vertical lines are the strings, while the horizontal lines are the frets. The lower pitched strings are on the left, while the higher pitched are on the right. Finger placement is indicated by the numbers written in the strings. The index finger is 1 and the pinky is 4. This diagram is of a G major chord and it uses three fingers, two on the third fret, and one on the second.
E A D G B e
0___________
| | | | | |
| | | | | |
| | | | | |
1-----------
| | | | | |
| 2 | | | |
| | | | | |
2-----------
| | | | | |
1 | | | | 4
| | | | | |
3-----------
| | | | | |
| | | | | |
| | | | | |
4-----------
However, you could also draw a guitar tab like this:
The above looks a bit more like the neck of the guitar from the point of view of the person holding it.
0 1 2 3 4
e|-------|-------|---4---|-------|
B|-------|-------|-------|-------|
G|-------|-------|-------|-------|
D|-------|-------|-------|-------|
A|-------|---2---|-------|-------|
E|-------|-------|---1---|-------|
One great way to learn how a musician plays an unusual chord, is to look at a video or picture. In that case, you'd see the neck of the guitar reversed, like this:
Unless, of course, you are watching one of the many left-handed guitar players (Hendrix, McCartney, etc.) in which case, you neck would look like this:
4 3 2 1 0
|-------|---1---|-------|-------|E
|-------|-------|---2---|-------|A
|-------|-------|-------|-------|D
|-------|-------|-------|-------|G
|-------|-------|-------|-------|B
|-------|---4---|-------|-------|e
The interesting thing about learning to play guitar using tabs, is that you actually have less information to go on than with sheet music. Tabs tend to lack information about the rhythm being played, tempo, and volume which are all present in sheet music. The one thing which sheet music lacks, however, is an indication of where your fingers should go when playing a particular chord or riff. Unpacking finger placement information from a cluster of notes on a staff can be difficult enough that guitar tabs make an attractive tradeoff. It is the musical equivalent of a domain specific language.
0 1 2 3 4
E|-------|-------|---1---|-------|
A|-------|---2---|-------|-------|
D|-------|-------|-------|-------|
G|-------|-------|-------|-------|
B|-------|-------|-------|-------|
e|-------|-------|---4---|-------|
With the prevalence of recorded music which can be rewound and replayed over and over, a guitar player can often reconstruct the rythms and other necessary information by listening to the song. No longer need all information live on the page, the quick and dirty guitar hacker plays with tabs on the stand and the music in her head.