First things first: What is a guitar tab?

Guitar tabs are a widely popular and easily producible form of guitar notation that helps you learn to play complex musical passages, like melodies or lead guitar licks.

The word "tabs" is short for the word tablatures. The very first tablatures were created for lutes back in the 1500s, no joke. Gradually, guitarists stole tablature for it's own use, and now it's used extensively to teach and to learn how to play. Here's what a couple of lines of guitar tab look like:

Free tabs that you find online might look a little choppy, but they get the job done:

Guitar tabs are not always accurate

Guitar tabs are usually made by people who just enjoy playing guitar and perhaps aren't professionals, and that's okay! Mistakes come with the territory, and that could be frustrating, but take heart: Remember that it's great that people are doing most of the work for you already. Any free guitar tab you get your hands on points to the most accurate way to play the song, but is not an authoritative source. 

In other words: Take any free guitar tab you find with a grain of salt, and then to find two more tabs to contrast and compare.

Learn how to read guitar tabs

Many beginners don't really get guitar tabs at first. Tabs aren't that hard to read once you get the hang of them, but be patient. It's takes a little bit of time to get used to them.

Where are the strings represented?

The strings are represented with six lines, like this: 

Notice how the thickest string, the string closest to the ceiling, also called the low E string, is at the bottom of the lines. This confuses a lot of beginners at first:

Also notice how the thinnest string, the string closest to the floor, also called the high E string, is at the top of the lines. This also confuses a lot of beginners at first:

What do the numbers mean?

Next, let's get into what the numbers represent. Many beginners make the mistake of thinking that the numbers represent the finger numbers. Nope! They are frets. If you see a zero on any guitar tab, that means you must play that string open, or "0" for open:

If you see a 0, 1, 5, 14, or even a 19 on a tablature, it means to play the 1st fret, 5th fret, 14th fret, or the 19th fret on your guitar:

Guitar tabs are linear. They represent time, just like a karaoke machine

Guitar tablatures start at one point in time and end up at another. Therefore, playing the following guitar tab,

...would sound like this:

Here's how I did it:

1. I first plucked the low E string
2. I plucked the 1st fret of the low E string
3. I plucked the 2nd fret of the low E string
4. I plucked the 4th fret of the A string
5. I plucked the 2nd fret of the A string
6. I plucked the open string of the A string
7. I finally plucked the 2nd fret of the low E string

I know what I just explained sounds so silly, but these notes are not played at the same exact time. They are played sequentially, one after another.** This is like the guitar version of karaoke. Play one note, play another, and not all at the same time or else it sounds like dribble. Get it?

This makes guitar tabs excellent for representing passages that are played sequentially, like a lead guitar solo, a fingerpicking pattern, or just a simple melody. Now, what are guitar tabs good for, anyway?

What is learning to read guitar tab good for, anyway?

Ask yourself: what exactly do you want to learn guitar tabs for? If you can't be explicit in why you need to learn them, you might not have to. I personally find it cruel and inhuman punishment to practice something that isn't appropriate for me to learn, so let's see if it's appropriate for you:

1. Acoustic Rhythm Guitar. It's not a bad idea to learn guitar tabs if you want to strum along with songs. Sometimes, you will spot small melodies or guitar licks that you'll want to play, and tablature is the best way to learn how to play them. 
2. Electric Rhythm Guitar. Absolutely. Sometimes you'll need to learn melodies that can't be explained via guitar chord diagrams. 
3. Lead Guitar. Yup. No doubt. Do it.
4. Blues Guitar. Ditto. No doubt about it. Do it. 
5. Fingerpicking Guitar. Many times, you'll see fingerpicking guitar patterns written out with tab, so learning guitar tabs is required.
6. Jazz Guitar. I'd say that it isn't a bad idea to read guitar tabs if you want to learn jazz, though it probably won't have too much of a direct benefit on your enjoyment of playing music. It's simply another way to communicate with other guitarists who have a different skill set than you.
7. Classical Guitar. I know that many classical guitar publishers are hip enough to include tablature as well as guitar music notation in their instruction books. However, any time that you spend away from learning guitar music notation is time wasted. Ignore guitar tabs altogether.

*Many guitar tabs clearly state that they are to be used for educational and personal study reasons. This one clause in copyright laws allows free tab hosting sites more leeway, and it makes them less likely to receive cease and desist orders from the copyright owners, or the RIAA for that matter.

**Chord diagrams are a little different than guitar tabs. They are not linear. They will tell you where you must place your fingers, and then you'll hold it in this spot while strumming. 

Photo Cred: Guitar Wolf by Masao Nakagami, Attribution-ShareAlike 2.0 Generic