As the name suggests, it's a situation where a character's gender is switched.
Manga and anime that focuses on genderbending usually involves a story of a boy being turned into a girl and vice-versa through a comedic and/or magical accident/method. Such manga usually deal with the comedic or sexual subject of having ones gender reversed. Series like Kampfer, Ichinensei Ni Nacchattar, and others deal with the sexual and comedic side, while series like Kashimashi ~Girl Meets Girl~ deal with the romantic aspects. Some series use genderbending as a single-time gag or for comedic effect. Naruto has a technique in which he appears to have changed gender. Genderbending manga and anime also deal with sexual identity, usually deailing with yuri or yaoi situations. Doujinshi often swap genders for humor or hentai means.
Other anime with genderbending themes include: