Warriors summoned for the Holy Grail War in Fate/ Stay Night. Legendary heroes of the past, now Half-spiritual beings with extreme power.
Servants are Heroic Spirits summoned by the Holy Grail. Once summoned, they can be classified as familiars as long as they are bound to a Master. Heroic Spirits are legendary heroes of the past( sometimes of the present or the future too, altough it's rare if ever happens) who are accomplished almost impossible deeds and ascended to the Throne of Heroes, a realm outside of space and time. From then on they're summoned at great calmities to save the world once again.
Servants are the copies of those souls with slightly inferior abilities. Because summoning even one heroic spirit is a miraculous occurence, summoning seven in their pure forms would be beyond the Holy Grail's power. Thus, to facilitate the process, Servants are summoned into one of seven vessels prepared beforehand. These vessels are classified based on their primary roles. Heroic Spirits can be summoned into any "class" that reflects the abilities they had in life. The seven standard classes are Archer, Assassin, Berserker, Caster, Lancer, Rider and Saber. According to Rin in the Fate Route, servant classes are changed between each war.
Although too powerful to be controlled by humans, the prospect of having a wish granted by the Holy Grail serves as incentive cooperate with a Master. A Master who is chosen to participate in the Holy Grail War possesses the Command Spells that, when invoked, forces the Servant to obey the Master's orders. However, few Servants actually appear to have a wish, with only True Assassin and Saber expressing theirs while the others are either content to serve their Master or have other goals entirely.
In order to call forth a specific Servant, a material connection between the summoner and the Heroic Spirit is required. This is
particularly true if one wishes to summon an exceptional Servant. For example, the Einzberns used Avalon the sheath of Excalibur, in order to summon Saber and a slab of rock from a temple to summon Berserker in the fourth and fifth Holy Grail Wars respectively. If no material component is provided, a Servant with a character similar to the summoner is summoned.
Servants can revert to spirit form at will, and in this form they can pass through inorganic substances that aren't dense with mana. When Lancer invaded the Emiya mansion, he used his spirit form to infiltrate the house. He subsequently reverted to physical form to drop from the ceiling into the living room.
They cannot be affected by normal interference when dematerialized, but need to materialize for battle.
It is to be noted that originally, the Servant system was designed to summon only the souls of "true" heroes - benevolent, honorable, and selfless protectors of humanity and justice. But since the corruption of the Holy Grail in the third Holy Grail War, evil spirits can be summoned. These entities can still be considered to be Heroic Spirits if they were widely worshipped, respected, or feared in life.
Finally, Servants cannot exist outside of the Holy Grail War, because Grail is one which supports their existence.