The seventh chords have in common that the seventh note in a scale is added to a triad. We can compare the different seventh chords in the note of C:
C7: C – E – G – Bb
Cm7: C – Eb – G – Bb
Cmaj7: C – E – G – B
The name "dominant" refer to the fifth degree of the diatonic scale and it is called dominant because it is most weighty besides the tonic (the root of the scale). The most common dominant chord is a dominant seventh in which a minor seventh is added to a triad major. A less common alternative chord name for C7 is Cdom7 (dom stands for dominant). The seventh chord belongs also to the group of extended chords.
Minor 7th is constructed by adding a minor seventh tone in the scale to a minor triad.
Major 7th is constructed by adding the seventh tone in the scale to a major triad.
Additional seventh chords
Here are the other three groups of seventh chords, with examples in the note of C:
Cdim7: C – Eb – Gb – A
Caug7: C – E – G# – A#
CmM7: C – Eb – G – B
For more variations of seventh chords, see also Altered chords.