Of all the chords to learn about, minor chords are one of the most important. They play a central role in music theory and chord progressions, and can be found in pretty much any song or musical piece you take a look at. They sound sad, moody or dark, as opposed to the more cheerful sound of a major chord.
A minor chord is made up of three notes. They are the first, third and fifth notes of a minor scale. Because of this, the three chord tones are called the "root," the "third," and the "fifth." In between the first two notes is the musical interval of a minor third, and in between the last two is a major third.
The frequencies of the three notes in a minor chord line up with each other in a 10 to 12 to 15 ratio, creating a nice harmony. That is to say, for every 10 vibrations of the root note, there are about 12 vibrations of the third and 15 of the fifth.