Chords keys. Of more relevance to the guitar player are ‘chord families’, or the groupings of chords that go with the keys. This could be useful in knowing what chords actually sound good together.
This easy-to-understand theory will help you to compose your own songs, obviously, but not only this! You will have more tools to play those songs where you don't exactly remember the chords, or even to guess the chords of a song you like.
According to the C scale, each note can represent a specific chord in the key of C. Look at this chart of chords for the key of C: