Like the Open D tuning, the Open G tuning has been around for well over a century. It has been called the "Spanish" tuning because back in the 1800's a very popular parlor piece called "Spanish Fandango" was played in this tuning. The history of "Spanish Fandango" itself goes back at least 140 years. In fact nineteen different arrangements of this melody are known to have been published before 1897; all in Open G. Since then, this tuning came to be used in numerous musical styles from blues guitar to Hawaiian "Slack Key". The Hawaiian Slack Key players had there own name for this tuning; they called it "Taro Patch". The reason blues guitarists liked Open G is for the same reasons they liked Open D, the ability to lay a slide across all the strings and sound a major chord.
Like Open D, the Open G tuning is a major chord tuning, but in the key of G. They have the same note relationship between strings as Open D, but shifted one string toward the treble. Where Open D has the root note of the D scale on the Open 6th string, the Open G has the root note on the Open 5th string. So the relationship of the notes from the 5th string to the 1st is the same as the relationship Open D has from the 6th string to the 2nd. Notice below how the I-V-I-III-V pattern for Open D occurs in Open G, but starting from the 5th string.
Open D String 6 5 4 3 2 1 D - A - D - F# - A - D I V I III V I Open G String 6 5 4 3 2 1 D - G - D - G - B - D V I V I III V
The reason this is useful to note is because chord patterns learned in Open D can be used in Open G; just moved down one string.
An important thing to note in the Open G tuning is the fact that the Open 6th string is not the root note of the G scale; its the fifth of the scale. When all the strings are played open, the chord form is actually Gmaj/5. If you remember from your chord theory, the "/x" represents what note of the chord is in the bass; in this case, the 5th of the chord is in the base. To obtain standard a Gmaj chord with the root in the base, the 6th string would need to be muted. So what you loose with this is some of the rich harmonic resolution you can get by having the root note be the lowest note on the guitar; but this chord voicing can, and has been, used to advantage in chord progressions and turnarounds. When choosing which tuning to use, Open D or Open G, this difference is one the key things to keep in mind.