Warriors summoned for the Holy Grail War in Fate/ Stay Night. Legendary heroes of the past who are 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, although it's rare if ever happens) who have accomplished almost impossible deeds and ascended to the Throne of Heroes, a realm outside of space and time. From then on, they are summoned at great calamities 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 occurrence, 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 one, 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, 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.
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 Heroic Spirits if they were widely worshiped, respected or feared in life.
Finally, Servants cannot exist outside of the Holy Grail War because the Grail is the one which supports their existence.
Servants of this type are known to be skilled in the use of swords and have high attributes at all statistics. They have high Magic Resistance and Riding skills, as well as being known to deliver powerful burst attacks with their sword.
Notable Saber-type Servant: Arturia Penndragon
Those of the Lancer class are skilled with long-ranged melee weapons like spears or lances and have high Agility ratings. The only notable skill they have is a high Magic Resistance skill, though it is normally not as high as the skill which those of the Saber-class have.
Archer-class Servants are proficient in the use of projectiles and having use of powerful Noble Phantasms, as their battle style mostly revolves around use of powerful projectile attacks. Archers are assumed by many to have Noble Phantasms related to a bow, though they don't necessarily need to be associated with one to be summoned so long as their fighting style revolves around use of projectile attacks.
They are notable for having the ability Independent Action, which allows them to travel freely for prolonged periods of time without requiring prana from their Master. This ability make them useful as scouts and allow them to retain their physical form for an extended period of time if the contract they make with a Master is severed. Those with Independent Action ratings of the highest level can be difficult to control as they are able to travel wherever they want and perform any action they please within their moral compass, with Masters usually having to resort to using Command Spells to rein control over them.
They also possess the Magic Resistance ability. But like those of the Lancer class, the ranking for it is not as high as those of the Saber class.
Rider-class Servants are known for having utilized use of mounts that are human constructs or living beasts throughout their lifetime. Their attributes are subpar, but possess a high Riding skill level that allow them to utilize the full potential of their mount which can include anything from simple beasts to even mythical creatures like a pegasus. Riders also have the Magic Resistance ability.
Servants of this type are known to be very proficient in the use of magecraft and possess two abilities that aid them in further enhancing their magic: Territory Creation, which alter the surrounding land to favor their magic, and Item Construction, which allows them to create magical items and tools. Beyond magic, Casters have weak ratings in other attributes and are known to be among the weaker Servants in the Holy Grail War since several Servant classes are known to have the Magic Resistance ability. However, it is possible for this preconception to be taken advantage of as the magical abilities of the Caster class can be underestimated if utilized properly.
Servants of this class are known to utilize stealth in their actions for battle. They are among the weaker Servants in the Grail War having low attributes, thus normal tactics of those in this class are to kill off Masters instead of Servants. Assassins possess the ability Presence Concealment, which allow them to completely mask their presence to others while they observe or plot to ambush their targets. Without a Master, Assassins are able to remain in the world for about 40 hours.
Servants of the Assassin class are normally assigned to those known as Hassan i Sabbah, the various leaders of a medieval Islamic cult called Hasshashin. However, this rule became tampered with when Medea was able to summon her own Servant to fill the Assassin class; one whose attributes, abilities and tactics differed greatly from those normally assigned to the Assassin class.
Berserker-class Servants are assigned to those who had periods where they when into berserk rages at least once during their lifetime. They possess the ability Mad Enhancement, which allows them to gain significant boosts in their attributes at the cost of their consciousness and sanity. The loss of sanity prevents a Berserker from having complex thoughts such as utilizing battle tactics and reasoning, but blesses them with monstrous strength that they can utilize to overwhelm their foes and it is possible for the Mad Enhancement ability to make a Berserker much stronger than they were in their time alive. Setbacks with Berserkers are that they require large amounts of mana to utilize the significant power they make use of and their insanity can make it difficult for a Master to maintain control of them.
Avenger was an improper class summoned by the Einzberns during the events of the third Holy Grail War. The creation of this class was an attempt by the Einzbern family to summon the Zoroastrian god, Angra Mainyu, out of their disgust for losing the prior two Holy Grail Wars. This class replaced the Berserker class for the Grail War, but resulted in the creation of a Servant with weak attributes and no special abilities. Avenger was absorbed into the Holy Grail upon his loss, which resulted in its corruption and the rules of summoning a Heroic Spirit being altered as villains and anti-heroes could now be summoned into later Holy Grail Wars.
Notable Avenger-Class Servant: Angra Mainyu