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Seiyuu (声優 seiyuu, also spelled seiyu or seiyū) is the Japanese word for a voice actor. Seiyuu do narration, radio, video games, and Japanese dubs of foreign movies, but in English-speaking fandom they are best known as the Japanese voices in anime shows and movies. English-speaking fans typically use "seiyuu" to refer to the Japanese VA, and "Voice Actor" to refer to the English dub VA.
As anime is so popular in Japan, there are many seiyuu, with over a hundred schools with voice acting programs, and the most popular seiyuu have full-time careers doing voiceovers. Many also crossover into music recording and become singers; some groups of seiyuu have bands tied into their anime series, such as Weiss Kreuz. With voice acting being such a major industry, seiyuu take their art as seriously as regular actors, and in anime will be as invested in their characters, and as influential in their portrayal in the show, pushing for particular nuances. Some seiyuu get a reputation for playing a particular type of role (e.g. bishounen).
Many seiyuu get fan followings, with fan clubs and more, among both male and female fans. In Japan, seiyuu live appearances and performances can draw thousands of fans, and seiyuu guest appearances at anime conventions in the Western world can be major events. There are livejournal communities dedicated to particular seiyuu, and some anime fans will follow seiyuu to new series, just as TV fans follow actors to movies or other TV shows; it's not unusual for an anime fan to start watching a series just because it features a favorite seiyuu.
See also the Wikipedia article on seiyuu.