Fanservice
Fanservice (or fan service) is a vaguely defined term used in visual media, particularly in the anime fandom (in Japanese, it is simply spoken as "service (saabisu)"), to refer to elements in a story that while potentially superfluous to a storyline, are designed to amuse or excite the audience. It is sometimes used in a derogatory manner when presented in a clumsy, pandering fashion. Since it is extremely subjective, the most common uses are listed.
Sexual
The typically understood definition is inclusion of racy or sexual content (usually female, but also male) to titilate the viewer, such as nudity. Shower scenes are very common in movies. Many television series use "obligatory" holiday episodes, especially to tropical locales, to depict characters in states of relative undress when it would otherwise be out of place with the tone of a series. In anime two common fanservices are the panty shot and the jiggling breasts, both often overused to an almost silly level (that is often what is desired). An example is the unfinished OVA Eiken.
Some series make jokes or comments that are only amusing to those "in the know," such as fans who receive club newsletters or perusers of Internet forums.
Technicals
Heavily used in much of science fiction, these are technical details sometimes bordering on arcane knowledge that hardcore fans are versed in, to show an author (often an admitted fanboy/fangirl themselves) pays attention to detail. For example, the recurring theme of well detailed and depicted automobiles in Oh My Goddess.
Current issues
Recently in anime shows there has been an explosion of shows that are criticized for simply being vehicles for fanservice. In contrast there has also been a rise in shows of the progressive anime category — shows which don't care so much about pleasing the audience but providing a full artistic vision. This has polarized anime to some degree making recent series either heavily fanservice-oriented (Green Green) or heavily experimental (Texhnolyze) with little in between.
