In the Major and Minor Chords section of The Basics you can see how a chord is essentially made up of three notes, and that some of the most common are major and minor chords.

Let's look at the major keys. There is a basic set of chords that goes with each key and these chords come from the major scale.The way this works is that you can build a chord on each note of the major scale.

Below is a scale of C with a chord triad built on every note. The root of each chord triad is a note from the major scale. The 3rd and 5th in the triad are the 3rd and 5th above the root as these notes appear in the key of C.

Every note in every chord is from the key of C. When chords just contain notes from the key they are called diatonic chords.