The four basic types

• maj7 (1 3 5 7): dreamy, at-rest, jazzy major
• m7 (1 b3 5 b7): smooth minor, no real tension
• dom7 (1 3 5 b7): tense, wants to resolve
• m7b5 / ⌀ (1 b3 b5 b7): half-diminished, in-between sound

Why dominants resolve

A dominant 7 chord contains a tritone (between the 3 and the b7) — the most unstable interval in music. In G7, that tritone is B and F. Both notes WANT to resolve: B → C (up half-step), F → E (down half-step). When the chord resolves to Cmaj7, both voices land in the most natural way. That's why V7 → I sounds inevitable.

Harmonized in 7ths

The 7 chords of a major key, stacked with 7ths: Imaj7 ii7 iii7 IVmaj7 V7 vi7 vii⌀7. In C: Cmaj7 Dm7 Em7 Fmaj7 G7 Am7 Bm7b5. Notice only the V7 is dominant — it's the only one with the tritone. That's why V is the 'engine' of tonal harmony.