Guitars are typical in Standard Tuning, which is E A D G B E.
Although if you play certain genres (generally heavier rock music), standard tuning may be pretty rare.
From the lowest (thickest string nearest the ceiling when you play) to the highest string (thinnest string closest to the floor), a standard guitar tuning is E A D G B E.
When tuned to standard, your guitar is tuned mainly in 4ths (if you count A as one and count up 4, you’ll get to D). The exception is the G and B, which is a major 3rd apart.
There are different theories about why we ended up with this exact tuning. It seems like it was popular because it was a good compromise. This type of tuning makes it easier to remember patterns for single notes, such as scales, but also to make chord shapes playable (without having to try and grow another finger).
‘Standard Style’ tunings
There are many reasons why you may want to play in a different tuning to standard.
Your voice (or your singers’ voice) range may struggle with some of the high notes. Some guitarists say they prefer the tone of the guitar when it is tuned down compared to standard tuning. You can also use it to add that heavy edge to rock music where the lower notes give the music attitude.
A common alternative tuning is Eb Tuning. You can think of this as a standard-style tuning as it is still the same intervals between each string, but with lower notes.
Eb Ab Db Gb Bb Eb
By using this tuning, you can play a low Eb chord, when before you were restricted to a low E chord.
Another use for this type of tuning is you can use all of your open chord shapes such as G, D, Em, etc.. However, they are now all Gb, Db, Ebm chords.
If you were to move the notes one more time again, you’d end up with a D standard style of tuning.
D G C F A D
You can keep detuning to whatever note you want, at least until your strings fall off.
Drop Tuning (Drop D Tuning)
In modern punk/metal music drop tunings are very popular. The most common drop tuning being Drop D as it is so close to standard tuning:
As they are so similar, this makes moving between the two easy in live settings.
Another popular drop tuning is drop C#, which is the drop tuning from Eb ‘standard’ tuning:
Advantages of drop tuning
You can now play power chords with one finger. By putting your finger across the 2 or 3 thickest strings (your choice), you are making a power chord.
Drop tuning can be useful if you want to move quickly between a chord higher up the neck and the low D5 chord (which is now the three thickest strings played open).
Disadvantages of drop tuning
6 string barre chords aren’t easily playable (which might not sound like a disadvantage to some people). Drop tuning generally make use of power chord-based guitar playing. The trade off is barre chord or open chord playing, as the shapes tend to become more difficult.