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.
In Evangelion, the episode preview for "Next time, service, service!"