"Waifu" is used to refer to a fictional girl or woman (usually in Anime, Manga, or video-games) that you have sexual attraction to, and you would even marry.

"Fan service" In general, fan service refers to scenes designed to excite or titillate the viewer. This can include scantily-clad outfits, cleavage shots, panty shots, nude scenes (shower scenes especially), etc. Some broader definitions also include things like cool mecha, big explosions, battle scenes, etc. Basically, if it has little plot-redeaming value, but makes the viewer  ...