To make a guitar sound good, the instrument needs to be in tune. Before you can tune a guitar however, you need to know to which notes the strings should be tuned. An electronic tuner is extremely useful when tuning a guitar, but so long as you have a reference point (a tuning fork or a note from another properly tuned instrument), you can tune relatively accurately by ear.
Guitar strings can be tuned to any note. The only thing limiting their range is the thickness of the string. A thicker string will snap if it is tuned too high and a thinner string will become floppy if it is tuned too deep. Despite this, a standard tuning has emerged that most guitar players use. Standard tuning is, from the thickest string to the thinnest, E, A, D, G, B and E.
While standard tuning is the most common guitar tuning, many alternate tunings are used by guitarists throughout the world. Some of the most common of these tunings include drop D (D, A, D, G, B, E), standard tuning dropped a half-step (Eb, Ab, Db, Gb, Bb, Eb), open D (D, A, D, F#, A, D), open G (D, G, D, G, B, D), open A (E, A, C#, E, A, E) and all fourths (E, A, D, G, C, F).