Chord types can be organized in many different ways, but one of the easiest ways to put chord types into separate categories is with the simple question, "major or minor?" Does anyone reading know why this is the case?
The answer is quite simple; at least, very simple to those who have an understanding of music theory. In jazz and to some extent in classical music, almost every chord has either a major or a minor tonality. For example, let's say you are in the key of C concert and you play on a piano the notes C, Eb, and G. You have played a C minor chord - in this case, the reason it's a minor chord is completely due to the Eb note. A C major scale includes the notes C, D, E, F, G, A, and B, but does not include an Eb - therefore, the notes C, Eb, and G can't be a C major chord of any sort.
But once we head into minor scales, all the notes of the above-mentioned C minor chord will be present - the C, Eb, and G. That's how we know the above-mentioned C minor chord really is a C minor chord. Meanwhile, a chord with the notes C, E, G, and B must be some type of C major chord because C, E, G, and B are all part of the C major scale. (By the way, the notes C, E, G, and B form a Cmaj7 chord.)
Dominant seventh chords technically are neither minor nor major chords, though, because a C7 chord includes an E (the E note is part of the C major scale, making the chord major). However, the C7 also includes a Bb - and a Bb note implies the chord is a minor tonality. Therefore, the C7 chord is given the name "dominant", which implies that it's sort-of major and sort-of minor. But the C7 chord is generally considered to be more like a major chord than a minor chord because the C7 chord's E note is more important tonality-wise than the Bb note.
There are also some chords that aren't really either major or minor, like diminished chords and augmented chords. However, in jazz these chords are not as frequent as major and minor chords, so we will not explain them in a detailed fashion today.
But what's so important about defining whether a chord is major or minor? There's not much difference in sound between these two kinds of tonalities, right?
If you went along with the two previous sentences, you were wrong. The difference between minor and major chords is one of the most important differences in music, particularly in jazz, mid-1900s popular music, and gospel music. Minor chords generally encourage thoughtful or sad emotions, while major chords encourage happier emotions. Here are some examples of this:
Stolen Moments, a tune written almost completely with minor chords that has a general minor tonality
St. James Infirmary, a minor blues
Yardbird Suite, a Parker composition that's written mostly with a major tonality (major key). Can you detect which part of the tune goes into minor?
Cherokee, another tune associated with Parker that clearly has a major tonality
Notice the clear difference of emotion between the first two above recordings and the second two - the first two recordings encourage sad emotions and thoughtfulness and the second two recordings evoke merry, almost bouncy emotions. However, keep in mind that the second two are played by Charlie Parker, who rarely, if ever, sounded sad. But these four recordings should give you the general idea.
These recordings should, therefore, make clear the difference between major and minor. Most tunes switch between these two types of tonalities, so the differences between major and minor are not so obvious to the casual listener as they could be. But remember that if music was all written in minor keys or all written with major tonalities it would not be nearly so interesting on an emotional level to any listener, even the most dedicated listeners and jazz fans.
Therefore, when musicians are putting together a repertoire or selection of tunes for any concert or performance and they want some variation between each tune, they should try to alternate between tunes with mostly major tonalities and tunes with mostly minor tonalities. Just think: playing Stolen Moments and then moving on to Cherokee would definitely provide varying emotional moods for an audience!