A very popular anime stereotype, usually, girls are tsunders, although there are some rare instances where otherwise is stated. Tsundere is a word made up from two other words, "Tsun Tsun" and "Dere Dere". Both terms describe acts of a person. "Tsun Tsun" is used for cold or rather blunt behaviour, meanwhile "Dere Dere" is used when a person becomes shy or flustered in front of their lover/crush.
How to know a character is a tsundere:
they will act mean to their crush while blushing or being flustered pouting/ "hmph!"
usually.. they're short
usually.. a girl
stomping
they're called, "Hiyoko"
A cheep imitation of a Tsundere. Used to describe a female anime/manga/game character with violent mood swings and a physically abusive personality. While a true Tsundere eventually warms up and calms down, even becoming quite nice near the end, a Counterfeit Tsundere does not and keeps her violent personality all throughout the show/book/game. The character's violence is mainly focused upon the main male character, though sometimes it can be targeted at others as well. Most of the anime Counterfiet Tsundere's are voiced by Japanese voice actress Rei Kugimiya. Basically a Counterfeit Tsunere is a girl with a bi-polar disorder.
Examples of Counterfeit Tsunderes include Shana, Louise Françoise Le Blanc de La Vallière, Nagi Sanzen'in, Taiga Aisaka, Naru Narusagawa.