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 sit up and take notice, it's probably fan service in one form or another.

see fanservice

it's about funny people dressed in clothes doing stuff that some other people like to see, frankly, i'm not really into it man but hey, whatever floats dawg!