Servants are heroic and divine spirits fighting in the Holy Grail War. They are summoned by their Masters.
Standard classes:
Three Knight classes
Saber
Lancer
Archer
Four Cavalry classes
Rider
Caster
Assassin
Berserker
Non-Standard classes:
Gilgamesh
Avenger
Shielder
Saver
Ruler
Beast
Moon Cancer
Alter Ego
Launcher
Watcher
Faker
Non-official classes
Boxer
Butler
Gorgeous
Idol
Monster
Tiger
Notes
although Gilgamesh is mostly considered as Archer-class Servant, he doesn't fall under any category in Fate/Extra
Saver is not to be confused with Saver from Fate/Prototype who is a Saber-class servant
Non-official classes are titles designated as nicknames or jokes in the several Fate titles. They are listed here for the sake of completeness, but there is no need for a tag.