Standard guitar tuning is quite simply the most common tuning you're going to come across on the guitar. If you were to go into any guitar shop, there's a 99% chance that any guitar you pull off the shelf will be tuned to standard guitar tuning.
With that in mind, it is important to note that there are literally hundreds of alternate guitar tunings; your own creativity is the only real limit on what can be done in this area.
For most beginner guitar players though, and for that matter, most other guitar players of all skill levels, standard tuning is pretty much the only tuning that matters. As such, it is extremely important to know what it is. If you don't know what the note is supposed to be, how will you ensure that your guitar is tuned correctly before practicing or performing?
I know, it seems like a basic thing that I may be harping on a bit much here, but honestly this is of such fundamental importance that it really does bear mentioning. I've seen people playing out of tune guitars, or wrongly tuned guitars, and hey, it ain't pretty. Then they say they don't really enjoy playing that much, or that no one likes listening when they play!
No wonder eh?
There isn't any great mystery about standard guitar tuning; it is E A D G B E. The 1st string (the one closest to the ground, and highest in pitch) is the last note in that list (E). That's right - when we talk about the tuning of strings and their note names, we talk about them from lowest to highest in terms of pitch, not physical position. This can get a little confusing for beginners, because the string numbers are the opposite: they start lowest to the ground, and progress upwards to your face.
So in terms of string numbers and names, standard tuning looks like this 6-E, 5-A, 4-D, 3-G, 2-B, 1-E. As you progress from the 6th string up towards the 1st, you'll rise two octaves in pitch. So the high E (1st string) is two octaves higher in pitch than the low E (6th string).
Hopefully by now that is abundantly clear, but if there are any remaining questions, have a look at the short video below, where I explained these same things.
Standard Guitar Tuning
Watch on Youtube