Well, back in the day girls had a lot of quirks but none were actually implemented, which was confusing for the players since there was no visual difference between ones you haven't unlocked yet and ones that simply aren't in the game.
Now all of the quirks you see are the ones you can get.
This, of course, has left some girls without any of them.
As for what they are?
Pretty much just achievements for doing certain things with some girls.
You probably don't want to unlock all of them so don't feel pressured.
I personally don't have any interest in unlocking Gwen's "damaged".
In the future, the idea is to have them gatekeep some content.
Like, taking Jessica's "Adultress" quirk as an example.
Unlocking it would tell the game you like that sort of content, which would maybe unlock more scenes like the one you unlock the quirk on.
If you don't have the quirk, that content will either be disabled or less visible.
But currently they're just achievements, if anything.