These are the traditional qualifiers for chords.
In functional harmony (as used for example in jazz), one usually distinguishes only 3 functions for chords (tetrads): tonic, subdominant, and dominant. In major harmony, tonic chords are those wich contain neither 4 nor 7 (the leading tone), subdominant chords contain 4 but not 7, and dominant chords contain both 4 and 7 (which form a very unstable tritone).
In C major this is:
- tonic: Cmaj7 (or C6), Em7, Am7
- subdominant: Dm7, Fmaj7
- dominant : G7, Bm7b5 (less frequent as a dominant chord), Bo7 (not diatonic in major, but frequently used as a rootless altered dominant, i.e G7b9 without the G).
Chords of a given family can be easily substituted to each other without changing their functional role in the harmonic progression (because they share 3 chord tones).