Play G one string at a time and you hear G, B and D. G is the root, B the major third and D the fifth.
From the low string to the high one you hear G, B, D, G, B and G. The root, G, shows up three times in the main shape. The lowest and highest notes are both G, two octaves apart, on strings 6 and 1.
G lets three strings ring open. String 4 gives D, string 3 gives G and string 2 gives B. Everything fits between frets 2 and 3. Besides the main one there are three more shapes, at frets 3, 5 and 10.
G7 is one change away. String 1 moves from fret 3 to fret 1, bringing in F, the minor seventh of G7. It shares two notes with Em, G and B. The change that keeps the most fingers still is to Em. One stays in place.
G shows up in seven keys. In G major it is the I, in C major the V and in D major the IV. The rest appear in the key list. Further down there are three songs that use it.