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"Tsundere" is a portmanteau of two Japanese phrases: "tsuntsun" and "deredere" and refers to characters who act hostile to the object of their affections, but secretly (or not-so-secretly) harbor feelings for them.
In a more practical sense, tsundere refers to a character who attempts to hide their feelings for another under a mask of indifference or even dislike. A "tsunderekko" is a tsundere female; more rarely you may see "tsunderekun" for a tsundere male.
The term and character archetype has existed for quite sometime within the realm of anime and manga, but was popularized relatively recently with the release of Ken Akamatsu's harem romance/comedy Love Hina. It was through this series in particular that the world was introduced to its tsundere icon in Naru Narusegawa, who found herself so overwhelmingly embarrassed by her apparently unwilling return in affection toward Keitaro Urashima that she spent the better part of her years living with the ronin verbally abusing and physically assaulting him for some of the most obviously innocent mistakes and confessions.
There's also the example of Shana, the title character of Shakugan no Shana, whose feelings toward her partner and friend Yuji Sakai compel her to go above and beyond her own call of duty to keep him out of harm's way. This unnatural (for her) compulsion leads to often heated and violent arguments between the two characters, and her self-consciousness over her flat chest -- a physical feature becoming increasingly common throughout the archetype -- often leads to embarrassing situations for which Yuji ultimately pays, regardless of the extent of his involvement.
A fitting example of the "tsunderekun" or male tsundere role would be Naoki Irie from the acclaimed romance series Itazura na Kiss. The character knowingly responds to the desires of Kotoko Aihara, who perplexingly wants nothing more than to be by his side for the rest of their lives. The problem is that Naoki is generally cold-hearted and self-centered, and spends the vast majority of his time around Kotoko being overly critical and ridiculously harsh with regard to pretty much everything she says and does.
The voice actress Rei Kugimiya is likely most associated with anime involving characters of this time, her having portrayed Shana, Louise de la Valliere, Nagi Sanzenin, Yuuhi Katagiri, and quite a few others.
Somtimes kuudere characters are confused for being tsundere, the major difference being tsundere are usually more denying and abusive whereas kuudere usually don't deny liking someone when asked, and they are rarely as abusive as tsundere.