Ask ten anime fans what fanservice is, and you will probably get eleven answers. Fanservice is basically whenever a series inserts a reference that is unnecessary, and therefore done simply to please the fans. This can come in the form of an in-joke, a technical explanation in a science fiction anime, a cameo, or a creator’s homage to a classic series. However, most commonly, fanservice as a term is used to describe the unnecessary attractive poses of characters to please the fans.
Often times, the viewers of a series will see a filler episode or two that simply exist as works of fanservice. These are usually placed in after a major arc has ended. Likewise, an anime series that is being published currently with a manga series will sometimes have a waiting period while the manga catches up to the anime’s production schedule. Bleach, especially, has had to insert filler story arcs to allow Tite Kubo to catch up his drawings with the anime release.
Having characters in bathing costumes is quite popular for this fact. In the past, there were constantly showcases of pools and beaches to work characters into swimsuits. However there have also been trends to showcase characters in traditional Japanese hot springs, called onsen, for the same reason.
Fanservice can actually be written into the plot, despite many anime directors having a penchant for using it in the middle of a storyline in such a manner that it does not otherwise fit. The somewhat misanthropic heroine Misa Amane is captured by the detective L, who had her placed into a straightjacket. Of course, the directors of the series waste no time in giving the viewers many shots of this scene.
Harem anime series are particularly notorious for their heavy amounts of fanservice. Any series in which a single male character is surrounded with beautiful women, who often are connected to him in some strange and surreal way, is considered to be a harem anime. Once again, the directors use this sort of opportunity to pepper the show with attractive shots of its characters.
Fanservice is a constantly growing and evolving trend in anime and manga. Many fans of the Neon Genesis Evangelion franchise may look back at the Misato character promising that she would offer service in the next episode. Some fans have described the entire opening seen of Lucky Star as a work of fanservice unto itself. In fact, there is a whole subgenre of anime called Ecchi that includes series, which are built around fanservice scenes. Whether one likes it or hates it, fanservice shows no sign of slowing down.

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@anime
Thanks for sharing your appreciation for harem anime ;-)