Fret spider nailed the essence of it by pointing out that any shape containing the notes G, B & D make a G major chord regardless of where you happen to play those notes. (Good job!)

I want to add a further clarification that has to do with voice leading.

If you play a G major with the D on the B-string and then ...

Originally Posted by: findinghomersame with C add 9. ive noticed it usually in songs that follow with a d chord,,, is this just to switch to the D easier or something by having the ring finger already on that note? ...

... all three chords have that one note! You aren't missing anything, you made a good observation. It can be physically easier to have an "anchor" note to hold to throughout the chords and it is a common tone that all three chords share to create a unified sort of effect.

Alternately, you can play a G major with the B string open, then play the C major chord with the C on the B string and then the D major chord with the D on the B string:


E |--3--3--2----|----3--0--2----|
B |--3--3--3----|----0--1--3----|
G |--0--0--2----|----0--0--2----|
D |--0--2--0----|----0--2--0----|
A |--2--3-------|----2--3-------|
E |--3----------|----3----------|


Unlike the first example, this creates a little voice motion and melodic thread on the B string that wasn't present before.

Essentially, the point I am trying to make here is to pay attention and notice these little things that can have such a profound different effect on the music you play.