The open G tuning has D at the bottom of the chord, creating an inversion. Some guitarists feel produces an awkward, "bottom-heavy" sound and will remove the low D string from the guitar and only play with five strings.
Keith Richards of The Rolling Stones is famous for introducing the the 5-string version of the open G tuning to rock music on songs such as "Brown Sugar", "Honky Tonk Women", "Wild Horses", and "Start Me Up".
Dobros use a full six-string tuning with a bottom G: G-B-D-G-B-D, low to high. But the Russian seven-string guitar uses an open G for its standard tuning and has a D at the bottom, it progresses D-G-B-D-G-B-D, low-high.
The 5-String Banjo's standard tuning is also an Open G: g-D-G-B-D, where the lower case "g" denotes the highest-pitched string.
Open G is popular for slide playing, but be aware non slide applications tend to be more difficult than other altered tunings. For beginners Open D and Modal tunings could be easier places to start.