For More information on what a Servant is please read the article "The Basics"
When a servant is summoned it is automatically categorized into one of the seven classes. They will most likely call themselves by their class rather than their real names because it would smarter to hide their true identity.
THE SABER, Knight of the Sword
Saber, Knight of the Sword, is one of the seven normal Servant classes summoned for the Holy Grail War and considered one of the Three Knight classes along with Lancer and Archer. Servants placed within this class are agile and powerful melee warriors adept at swordsmanship. The qualifying conditions require the Heroic Spirits to have legends as a knights of the sword, and it is the class that requires the highest levels of attributes in all but the Magic attribute. The Saber class has, without fail, proceeded to the final battleground in every Holy Grail War, so it is boasted as the strongest among all classes.
Class Requirements:-High Attributes in all classes
Class Benefits:-Strongest Magic Resistance of all the classes
-Riding ability
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THE ARCHER, Knight of the Bow 
Archer (アーチャー , ?), Knight of the Bow, is one of the seven normal Servant classes and it is placed among the Three Knight classes along with Saber and Lancer. Servants placed within this class are usually proficient with projectiles and excel in possessing powerful Noble Phantasms. More so than any other class, their true strength and the qualifying condition to be placed in the class is determined not by their attributes, but the possession of powerful projectile weapons or special abilities related to projectile weapons. Their main class ability is "Independent Action", the ability act to independently for a period of time without an active supply of prana from their Master. This makes them suitable as scouts, and it allows them to survive for a duration of time if their contract with their Master becomes severed with more time granted by higher ranks. Those with the highest ranks can be difficult to control as the ability allows them to travel where they please, and do as they wish, without anything besides their own moral compass and the threat of Command Spells to restrict them. They also have the class ability of Magic Resistance, but it is not as great as that of the Saber class. Although Servants of the Archer class will generally be assumed to have Noble Phantasms relating to a bow, this is actually usually not the case.
Class Requirements:Class Benefits:-Riding ability
-Magic resistance
-Independent Acton
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THE LANCER, Knight of the lance
Lancer, Knight of the Lance, is one of the seven normal Servant classes summoned for the Holy Grail War and considered one of the Three Knight classes along with Saber and Archer. Servants placed within this class, many of which are Heroic Spirits who were knights, are very agile, proficient in hit-and-run tactics that capitalize on range and speed, and skilled with long-range melee weapons such as spears and lances. The qualifying conditions to be placed within the class are second only to the Saber class, requiring overall excellent attributes and a high agility attribute. Their only particular class ability is Magic Resistance and Battle Continuation.
Requirements:-High agility
-Overall well rounded in all attributes
Class Benefits:-Magic resistance
-Battle Continuation: The ability to keep fighting for a short period of time after a mortal wound has been dealt.
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THE RIDER Mounted Knight
Rider, Mounted Knight, is one of the seven normal Servant classes summoned for the Holy Grail War. Servants placed within this class are those known for riding mounts, living beasts or human constructs, during their lives. They generally have lower attributes compared to the Three Knight Classes, but the abilities of the mounts described in their legends compensate by granting them powerful Noble Phantasms or abilities. They have the class abilities of Magic Resistance and a high rank in Riding, allowing them to fully utilize the abilities of their mounts. Depending on the rank, they can take full control of everything from simple horses, modern machinery like motorcycles and planes, and even Divine Beasts.
Class Requirements:-Some Riding Ability
Class Benefits:-High Agility
-Highest Riding Ability
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CASTER The Magus
Caster, the magus, is one of the seven normal Servant classes summoned for the Holy Grail War. Servants placed within this class are adept in magecraft, and the only qualifying condition is the mastery of sorceries of the highest caliber compared to the specific parameter requirements of the other classes. Their class abilities is Item Construction, which allows the magus to create magical items and tools. Due to not requiring any strong statistics, they generally have low combat abilities, and due to the majority of Servants having some form of Magic Resistance, this class is thought to be the weakest of all seven. This perception can be made into an advantage, as Casters are sometimes underestimated, often to the misfortune of their foes.
Class Requirements:-Highest Magic Skill
Class Attributes:-Able to create magical items
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THE BERSERKER The Mad warrior
Berserker, Mad Warrior, is one of the seven normal Servant classes summoned for the Holy Grail War. Servants placed within this class are always incarnations of Heroic Spirits who have gone berserk during battle at least once in their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their consciousness and sanity for a large power boost. This class is usually for magi who have summoned Heroic Spirits with insufficient abilities and attributes, allowing them to strengthen their Servants to make up for their lower attributes and compete against stronger opponents. While it is ideal for Servants to reach the level of the Heroic Spirit, the class allows for their attributes to surpass even the original. If used on an already strong Heroic Spirit, such as Heracles, they will become empowered to truly extraordinary levels. The abuse of this to create powerful Servants has made the class referred to as the strongest of all seven classes.
Class Requirements:-None (Other than has gone mad)
Class Benefits:Mad Enhancement: trades the sentient mind of a servant in order to increase one's power.
Class Repercussions: Controlling a Berserker is a difficult task to do and requires either a great deal of prana or a highly skilled Magus as a master.
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THE ASSASSIN 
Assassin is one of the seven normal Servant classes summoned for the Holy Grail War. Servants of this class must be skilled at operating covertly, stealthy, and silently. This class is considered to be one of the weaker classes at fighting, as its members possess low attributes due to not having glorious legends as heroes, so it instead specializes in killing Masters rather than Servants. It is an oddity compared to the more formal knight classes that will attack using the most unfair and advantageous conditions, and with these conditions, it is possible for an Assassin to match and defeat other Servants. The Assassins' special class ability is Presence Concealment, which allows them to remain undetected when spying or preparing to ambush their targets. It can be said that the use of this ability during battle is the lifeline of Assassin. The greatest skill of each legally summoned Assassin is expressed as their Noble Phantasm.
Class Requirements: None
Class Benefits:-Conceal Presence
-Independent Action
