DADGAD is an alternate guitar tuning most associated with Celtic music. It has also been used in rock, folk, metal and other genres. It is a great tuning for fingerpicking and has lots of beautiful open chords.
Instead of the standard tuning (E-A-D-G-B-E) the guitar strings are tuned, from low to high, D-A-D-G-A-D, hence the name. Tuning to DADGAD is accomplished by tuning the first, second, and sixth strings down a whole tone (two frets).
The open strings of DADGAD tuning result in a D suspended fourth chord. A suspended 4th chord is neither major or minor and the tuning can be used for both.