tsundere
An insecure character type who alternates between a cruel (tsuntsun) and loving (deredere) personality.
Generally, her lover will do something to earn her affection, triggering a deredere reaction, but she will then compensate for this perceived weakness by going into tsuntsun mode, in which she will either deny or attack her lover. Tsundere characters are generally more comedic in nature than yandere characters, even though this character type can be played more seriously as well.
This tag, specifically, refers to girls displaying this personality via contradicting actions and emotions. The easiest example is offering a valentine's day gift, but telling the recipient something akin to "It's no big deal, I don't care about you, I just had some extra."
The original definition is a girl that starts out combative towards a guy but as they get to know each other, she changes to a deredere personality and generally staying there. The other types came about as a result of various writers trying to copy it.