Tablature

I'll keep this relatively brief, as I'm only familiar with the basics.

Tabs are a form of musical notation that tell you where to place your fingers (rather than just telling you what to play, like sheet music, tabs also tell you how to play it). It looks similar to sheet music too, to the untrained eye.

Tabs are written like this:

e|--------------------------------|
B|--------------------------------|
G|--------------------------------|
D|--------------------------------|
A|--------------------------------|
E|--------------------------------| <-- Thickest (top) string

Each line represents a string, with the bottom line representing the top string. This sounds counterintuitive, but makes sense, considering the top string is closest to you (or, your eyes at least) when you are playing. The letters on the left indicate what note that string should be tuned to. Different songs require different tuning.

Here is the tab for the intro to 'Come As You Are' by Nirvana, which is nice and simple:

d|--|-------------------------------------------------------------| 
A|--|-------------------------------------------------------------|
F|--|-------------------------------------------------------------|  
E|--|-------------------------------------------------------------|
G|--|-------0---0---------2-----2---------------------------------| 
D|-0|0-1-2---2---2-2-1-0---0-0------------------------------------|

As you can see immediately the letters on the left are different to those above, because this song requires different tuning. You will also note there are numbers on two of the 'strings', ranging from 0 to 2.
The number corresponds to the fret that that note is played on. So 1 = first fret, 2 = second fret etc. 0 denotes an 'open' string, meaning that you do not hold it at any fret, or indeed at all.
There is also a second line right after the first note is played - whoever wrote this tab put that there to remind us that in this song that first note is only played the first time, so after that we start from that line rather than the first one.
So, the above tab means:

"Pluck the top string (D) 'open' twice, then once at the first fret, and once again at the second fret. Then pluck the second string (G) 'open' once, the first string (D) on the second fret, second string (G) open again, first string (D) second fret twice, then first fret and then open. Then the second string (G) on the second fret, the first string (D) open twice and then finally the second string (G) on the second fret again."

You can see why it was necessary to find a way of writing how to play that was shorter than that!
There are plenty of other symbols you can find on tabs, but as a beginner I don't really need to worry about them yet.

Hopefully that should be enough for you to get started. If you want to read more on tabs check out the following links:

• Wikipedia entry (guitar tablature is half-way down the page)
• How To Read Guitar Tabs (more detail)
  

You can find thousands of tabs online, some easier to read and play than others. Try googling some of your favorite songs!