Open G tuning
The open G tuning is popular among guitar players who, for example, favor blues styles. It gives you a slight different sound than normal. The many straightforward moveable shapes make the chords very easy to memorize and it also gives chromatic possibilities.
To get the open G tuning on your guitar you should tune it like this: D G D G B D.
Major chords (moveable shapes)
All you have to do is to lay your fingers over the strings and avoid playing on the lowest string (for the D major in open G tuning you could also include the lowest bass string). As the pictures illustrate the G chord could be played with all open strings.
The shapes are moveable, so if you move the finger two steps up the fretboard from D major you will get an E major and so on.
All common major chords in open G:
F: X10 10 10 10 10
7th chords (moveable shapes)
The seventh chords are also moveable and you just add another tone in front of the barre finger three frets up. To get a maj7 chord, just add the finger on the highest string one fret up.
Minor chords (moveable shapes)
While major chords are extremely easy in this tuning, the minor chords are trickier. You probably recognize the Am-shape here. Besides the fingers in the Am-shape you add two notes on the bass strings and this you do by placing the thumb over both the strings. This is probably the most efficient way to play minor chord in this tuning, but if you prefer you could also do it by using another moveable shape without the thumb involved: X22212. In this case you use your long finger for pressing down three strings.
Sus and add chords (moveable shapes)
The same method is used once again for sus and add chords. To get a Bsus4 just move the whole shape one fret up the fretboard. The same thing with Badd2.
Open chord shapes
Finally, let's look on some open chords in open G tuning.
This was an introduction about chords in open G tuning. This video could be recommended if you want to learn more chords and ideas on how to apply them.
See also open D tuning and drop C tuning