Tablature (tab) is the simplist form of music notation on the guitar. This is how it is usually laid out...

e|-------------------------------6th String-|
B|-------------------------5th String-------|
G|-------------------4th String-------------|
D|-------------3rd String-------------------|
A|-------2nd String-------------------------|
E|-1st String-------------------------------|

It is laid out like this because the higher up the string is on the diagram, the higher the note is. Here is an example of a simple tab...

e|--------0-----------0-----|
B|------0---0-------0---0---|
G|---0--------0---0-------0-|
D|--------------------------|
A|--------------------------|
E|-0------------0-----------|
Metallica - Nothing Else Matters

Listen

Tab is read from left to right. You play the notes in the order you see them. The numbers on the tab represent the fret you hold with your left hand while picking the string with your right. 0 represents an open string (No frets). Now try out the example below...

e|-----------------------------------------|
B|-----------------------------------------|
G|---------------------------------------0-|
D|---4---0-----4---0-----4---0-----4---0---|
A|-2---2---------------2---2---------------|
E|-----------3---4---------------3---3-----|
Metallica - One

Listen

The two previous examples used single notes. If notes are played together, they are written on top of each other and known as chords. Here are two examples of this...

e|---------| B|---------| G|---------| D|----5----| A|----3----| E|---------|

This shows two notes played together. You have to hold the position while picking the two strings at the same time.

e|----3----| B|----0----| G|----0----| D|----0----| A|----2----| E|----3----|

This is the G Major chord. You hold the above position on the fret board, while strumming all of the numbered strings with either a pick, your thumb or your thumb and index finger pressed together. Here are some more examples of this...

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

Sometimes, chords are written out like this...

022000

The number on the left represents the low E string, and the number on the far right represents your high E string. So on the example below, you would hold the 2nd fret on the 4th and 5th string and strum the lot. (The example above was the E minor chord)