Now, let's look at our G chord. For our stock open string G major chord, we can add in the D at the 3rd fret on the 2nd string. In addition, we can remove the high G at the 3rd fret on the 1st string. These chords then look as follows:
Since the D that we add is a part of the G major chord (the notes G, B, and D arranged in some fashion), our variations in Chords 2 and 3 above are still considered G major chords.
Listen to these variations below. I've included a strumming pattern to show it more in context. Note how subtle the difference is from one chord to the other.