Fanservice is a term that applies to any manga or Anime that has a lot of basis in the appeal of fan, usually erotic. Fanservice for males is often chracterized by an idealisticly pretty girl, often times with large breasts, very shapely figure and often putting a male and female into an erotic situation with each other (i.e. a night at the hotsprings)
An example of fanservice characterized by partial nudity
this can also be accomplished through full or partial nudity, or skimpy clothing.
Fanservice for females often lies in Yaoi manga, with much of the chracteristics only being applied to men instead of women characters. guys are very well toned and strong. Again nudity partial or full may be involved heavily.