"Tsundere" is derived from the words "tsuntsun" and "deredere," meaning "aloof, pointed, morose" and "exhausted, lovestruck" respectively. A tsunderekko (tsundere girl) can express this in one of two ways:
1) She is initially combative/aloof/cold, either towards the protagonist or everybody, and through the course of the story opens up and becomes emotionally vulnerable and loving (basically a personality shift through time).
2) She is combative/aloof/cold on the outside, but secretly she feels very vulnerable and loving (basically she puts up a farce to hide the 'real her').
This is a very broad character archetype that plays a variety of roles in anime and manga. From love interest to enemy to comedic relief, tsunderekko are versatile and dynamic, always doing something and certainly never being boring. Here is a place for us to come together and revel in all that is tsundere.(taken from MAL tsundere fanclub)