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 three 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.

When summoned, Servants are imparted with knowledge of technology, culture, science and terms of the era they were summoned into, as well as skill in at least the language of their Master, if not all languages. This is largely to prevent Servants from eras significantly before the era of a Grail War in question from suffering crippling culture shock which would otherwise prevent them from fulfilling their duties, and also enable them to blend with that era society. This can be understandable by such problems:
 * A Servant terrified of modern technology (such as electronic devices) and believes them to be sorcery.
 * A Servant cannot cope with modern era surroundings and thus suffers from depressions. This can also happen to a Servant that is summoned in his past life homeland and finds it is altered by modernization.
 * A Servant finds modern sciences ridiculous and even comical (such as the world is spherical and revolves around the Sun).
 * Limits the abilities of certain Servants. For example, an Assassin that is dumbfounded of modern security systems, a Saber, a Lancer and an Archer that can be easily defeated by modern weaponry, a Berserker that is depressed by modern surroundings and thus go on a rampage, a Rider who is clueless on driving modern vehicles and a Caster that is superstitious or easily fooled of or by electronic devices.
 * A Servant behaves in a manner that attracts the attentions of modern civilians, or using weapons against them, and thus apprehended by the authorities, or carted off to lunatic asylum.

Thus, without such knowledge, their potential would be crippled. This knowledge does not just cover history but all aspects of "modern" life during the Grail War in question, to the point that Servants can find things that would have been unthinkable in their lifetimes mundane or even worthy of scorn. As an example:
 * Arturia Pendragon of medieval era claims that she can fly aircrafts through her Riding skill. She is also able to ride a motorcycle and drive Irisviel's Mercedes without any difficulties and even able to enhance their potentials. In Fate/Stay Night she dons on a Japanese kimono.
 * Iskander of ancient Greece knows that the world is spherical. He also knows how to use modern money, order his T-shirt through postal service, buys a video games console and a video game CD, much to Velvet's chagrin.
 * Lancelot of the same era as Arturia's use a modern machine gun against Saber and is able to fly an F-15 and utilizes its armaments.
 * Diarmuid of medieval Ireland knows that Mercedes' engine that are drove by Saber is automatic just by hearing it sounds.
 * Medea of ancient Greece can be seen shopping at a mall and wears modern outfits. She also cooks with modern cooking utensils and appliances.
 * Cu Chulainn of medieval Ireland can be seen fishing with a modern fishing rods and wears modern clothing and runs a flower shop.
 * In Carnival Phantasm, Heracles of ancient Greece is instructed by Illya to buy her some batteries.
 * Gilgamesh of ancient Mesopotamia wear modern attires and able to blend with modern society without creating ruckus despite his violent and arrogance attitude.

Servants can revert to spirit form when not in battle at will to reduce the burden on their Masters. In this form they can pass through inorganic substances that aren't dense with mana; they are also invisible to the naked eye (except for the Master they connected with) and most forms of scrying in this state, facilitating the concealment of their identity. 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. As Human Spirits are they able to consume the souls and minds of people and convert them into energy. Their basic abilities do not change from such nourishment, but they become tougher and their magical energy capacity increases.

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; the strongest Archers can be difficult to control at times, in fact, due to their near-total independence from their "Masters". 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, which often include their mounts. Their special skill is Riding, which allows them to fully utilize abilities of their mounts (which can range from simple horses to mechanical vehicles to divine or supernatural 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 (although this rule does not hold absolutely true in the Fate/Extra reality). 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.
 * Saver: This is an irregular class in Fate/Extra used to summon Saver.
 * Temptress: An irregular class in Fate/Extra containing Arcueid Brunestud, classified as a creature which is a drain on blood, physical energy and money.
 * Monster: Non servant participant of Holy Grail war that appears in Fate/Extra.
 * Ruler: Servants placed in this class would have governed the Holy Grail War in the canceled Fate/Apocrypha project.
 * Beast: This class is a special container for the 666th Beast in Fate/Prototype.

Container Parameters
Unmodified by the strengths and weaknesses of specific Heroic Spirits, the containers of the seven standard Servant classes of the Fuyuki Grail War possess known base parameters, given below. Please note that the listed values exist only as rough references, and are non-absolute; variations can occur per summoning.

Known Servants

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

Holy Grail War
Some details of the Third Grail War are mentioned in Fate/hollow ataraxia. The events of the Fourth Grail War are described in Fate/Zero. The events of the Fifth Grail War are detailed in Fate/stay night and ''Fate/hollow ataraxia. ''If one looks carefully one can notice that six out of seven of the servents in Fate/Zero are all considered Knight Class servents.

Fate/Extra
The events of the Grail War of the Moon Cell are detailed in Fate/Extra and Fate/Extra CCC.

Fate/Apocrypha
Servants were designed for the Grail War detailed in the canceled Fate/Apocrypha project.