Minor 7th chords
Add a minor seventh to a minor triad and the chord's sharper edges soften. It reads as less dramatic than a plain minor chord but more colorful, and it's the backbone of the jazz ii chord in a huge number of ii-V-I progressions, along with most of soul and funk's chord vocabulary.
Other chord types
MajorMinorDiminishedAugmentedSuspended 2ndSuspended 4thDominant 7thMajor 7thMajor 6thMinor 6thMinor Major 7thDiminished 7thHalf-Diminished 7thAugmented 7thAdd9Dominant 9thMajor 9thMinor 9th6/9Dominant 11thMinor 11thDominant 13thMajor 13thMinor 13th7sus47b57b97#9Dominant 7#11Minor 6/99sus4Power Chord (5)Minor Add9Add117sus213sus4Minor Major 9thMajor 7#11Major 9#117b1313b913#117altAugmented Major 7thMinor 9b5Quartal (So What)