Servant

Nature
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. Servants are legends from the past, present, or future. 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.

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 a Command Spell 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.

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.

Classification
Heroic Spirits summoned by the Holy Grail are place into one of seven categories. The seven standard classes are Saber, Lancer, Archer, Rider, Caster, Assassin, and Berserker. An eighth class, Avenger, was created by the Einzbern family during the third Holy Grail War. According to Rin in the Fate Route, servant classes are changed between each war.
 * Saber: Servants placed within this class are agile and powerful melee warriors. It is commonly assumed to be the best class overall, with high ratings in all categories. It is one of the three Knight classes, together with Lancer and Archer and also possesses the Riding skill like the Rider class.
 * Lancer: Servants placed within this class are very agile and skilled with long-range melee weapons, such as spears, lances, etc. It is one of the three Knight classes, together with Archer and Saber.
 * Archer: Servants placed within this class are usually proficient with projectiles and can survive longer without a Master nearby, thanks to their special ability Independent Action. It is one of the three Knight classes, together with Lancer and Saber.
 * Rider: Servants placed within this class place emphasis upon speed and powerful Noble Phantasms. Their special skill is Riding, which allows them to fully utilize abilities of their mounts (ranging from simple horses, through motorcycles, to divine creatures).
 * Caster: Servants placed within this class are adept in magecraft and have a special ability akin to Reality Marble, called Territory Creation, which alters or creates space around them to enhance their sorcery.
 * Assassin: Servants placed within this class are always, as a rule, one of the 19 Hassan i Sabbah, the leaders of the clan of the Hashshashins who passed their name down.(The Assassin in Fate/stay night summoned by Caster is an exception, an aberration in the system, as he was summoned by another Servant.) This rule comes from how the word "assassin" itself finds its etymological roots in that clan. The Assassins' special ability is Presence Concealment, which allows them to remain undetected. Offset by their stealth is their weak combat skills. Due to this, Assassins often target Masters instead of Servants.
 * Berserker: Servants placed within this class are always incarnations of heroes who have gone berserk on their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their consciousness (i.e. sanity) for a large power boost. Most Masters are incapable of controlling their Servant once Mad Enhancement has been activated, which eventually results in their death.
 * Avenger: It is not one of the seven normal classes and was only summoned during the Third War.

Known Servants

 * Following list contains potential spoilers of individual Servants' identities.

Third Grail War
Some details of the Third Grail War are mentioned in Fate/hollow ataraxia.

Fourth Grail War
The events of the Fourth Grail War are described in Fate/Zero.

Fifth Grail War
The events of the Fifth Grail War are detailed in Fate/stay night and Fate/hollow ataraxia. Additionally, all servants from Fate/stay night appear in Fate/tiger colosseum.

Fate/Extra
The events of the Grail War detailed in Fate/Extra and Fate/hollow ataraxia.