In guitar playing, an open tuning is one where the strings are tuned so that a chord is achieved without fretting, or pressing any of the strings. With such a tuning, other chords may be played by simply barreing a fret or through the use of a slide.
Open tunings are common in blues music and some rock and folk music. They are particularly used in steel guitar and bottleneck guitar playing. The names of some tunings vary between genres, for example in Hawaiian Music, for slack-key guitar, an example would be the taro patch, or open G tuning, with strings low-high D-G-D-G-B-D.
But in bluegrass music, open G can mean G B D G B D
Major Tunings
Major open tunings (giving a major chord with the open strings) include:
Open A: low-high; E-A-C#-E-A-E
Alternatively: low-high; E-A-C#-E-A-C#
"Slide" Open A: low-high; E-A-E-A-C#-E (note that this tuning is identical to Open G tuning but with every string raised one step or two frets)
Open C: low-high; C-G-C-G-C-E
Open D: low-high; D-A-D-F#-A-D
Alternatively: low-high; D-A-D'-A'-D-D
Open E: low-high; E-B-E-G#-B-E (use light gauge strings because two strings must be raised)
Open F: low-high; F-A-C-F-C-F (rare)
Open G: low-high; D-G-D-G-B-D
dobro Open G: low-high; G-B-D-G-B-D (occasionally adopted for ordinary guitar, but requires lighter fifth and sixth strings;
Only square-necked resonator guitars can hold this tuning with standard gauges).
Russian Open G: low-high; D-G-B-D-G-B-D (the standard tuning for the Russian seven string guitar).
Open Tunings vs Altered Tunings
Generally, Open Tunings refer to the changing of string pitches to reproduce common Major and Minor chords. One might consider broadening this definition to include more obscure or less used tonalities / chords such as the ones listed below. But these are in kind of a "middle ground" between standard Open Tunings and Altered Tunings.
Altered tunings are tunings that don't really reflect any specific chord name. An example would be the tuning Jimmy Page uses on Led Zeppelin's Rain Song (D,G,C,G,C,D). Even though some tunings could be named by "theory" they might lack the gravity or musical cohesion to really represent that chord.