Three chords, and a thousand songs open up. In the key of G those three are G, C, and D — they sound like family because they're built from the same seven notes. That's all a key is: a family of chords that belong together. You already know these shapes.
Try this now Strum G, then C, then D, then back to G. Slow. Let each one ring. That little loop is the backbone of more songs than you'd believe.