Here are some of the most common symbols you'll find in modern tab. Check the glossary for definitions if you're not sure what they are.
Tablature is a way of notating music so that others can read it back and play it. It's not quite as exact as traditional notation (which I'm not very good at), but it's far easier to learn, as it's simply a representation of a guitar neck, the six horizontal lines representing the six strings. The graphic below shows which lines are which. The guitar is seen from the point of view of the player.
The thin string is at the top, the thick string at the bottom. The chord indicated in the image below is an 'A Major Seventh' chord, and the numbers on the TAB lines show that the treble string is played open (0), the next string down is fretted on the second fret (2), the next string down is fretted on the first fret (1) and so on. The 'X' means DON'T play that string. It really is a very straight forward, logical, system.
The time element of a piece of music reads from left to right, same as standard notation, and the numbers representing notes appear in the order they are played in. The chord in the example above would be played all at once, at the same moment, because they are all stacked up vertically.
These days, you'll find two kinds of tablature (usually referred to as 'tab', or tabs'). The older kind is 'ASCII Tab' which is simply text. It only shows the finger position numbers and the measures. Sometimes you'll see the names of chords written above the tab. Here is an example.
It's very basic. The notes are played in the order you're reading these words: from left to right. There is little indication of the timing other than some wider spaces, which hint at the fact that two of the notes last longer that the others ... the last notes of the second and fourth measures.
In the last few years, however, several Tablature software programs have been developed that come much closer to standard notation in the timing department.
You can save images of the rendered tab, as I have done for the many finger style lessons I post here at GfB&B. Here is 'Twinkle Twinkle', same as above.
This is much better. Here, you can see the time-signature (4/4), the measures are numbered and the stems below each note denote the time values for each. In this (very simple) example, quarter notes have stems that drop from just below each note; half notes have shorter stems that start below the lowest string line. A more complete list is at the bottom of this page.
And another look at that tune with a bit more detail added to the arrangement:
All the timing is the same as above, but here I've added some bass notes (circled in blue). The fact that they appear directly below the melody notes indicates that they should be played at the same time as those melody notes. If you play with your fingers, this is easy to do, just pluck those lower notes with your thumb at the same instant that your index or middle finger plucks the higher note. In measure 4, I've added a chord. Once again, the vertical stacking indicates that the notes are played at the same time. The software allowed me to add a diagram for that chord, showing you the familiar grid that is used for chords. This makes it a little easier to know what to do with your fingers. You can hear how much nicer this version is, just by adding a few extra notes.
Here are the symbols for the most common note duration values used in music. I've shown all the notes as C notes, 3rd fret of the fifth string, and below are the symbols for the Rests of that value. So the first, reading from the left, is a whole note, and below is a whole rest; next is a half note, below it is a half rest, and so on. There are also sixty-fourth notes and hundred twenty-eighth notes, but I doubt you'll be seeing them in any tablature anytime soon. At the far right are a set of triplets, which divide a beat into three. They use the eighth note symbol, but really they are 'twelfth' notes. Dotted notes increase the duration by half again, so, for example, a dotted half note lasts as long as a half note and a quarter note together. The rest symbols are exactly the same as standard notation and the note symbols are very similar. The lessons I post at this site use these symbols, which are generated form the Guitar-Pro software. I believe the other programs use the same ones.