Skills are the embodiment of a Servant's various abilities and characteristics. They are divided into two main categories: Class Skills, which are typically connected to the Servant's Class, and Personal Skills, which reflect abilities or anecdotes connected to the life and legend of the Servant themselves.
Certain Skills, such as Divinity, have been treated as both Class and Personal Skills depending on the work and occasionally between different materials for the same work.
Each Class is granted certain Skills upon summoning, particularly within the Holy Grail War system. They generally reflect abilities that the Heroic Spirit had in life, but they will not always have had the corresponding Skills. Even if the Servant did not have the abilities in life, they will still receive a lower ranked version of the Skill automatically if they fit the Class. Conversely, it is possible for a Servant of a different Class to gain any of these Skills as Personal Skills, as long as they reflect the abilities that they had in life.
Class Skills can be lost due to another Skill or Noble Phantasm of the Servant, or due to the Heroic Spirit's nature. The Skill Distortion, for example, forces a Class Change upon a Servant, granting them a Class Skill from the new Class in exchange for downgrading a Class Skill of the previous one.
Personal Skills reflect the abilities a Heroic Spirit had in life and their personal legends. As they are not strictly symbols of Heroic Spirits to the same extent as Noble Phantasms, there are many cases of Heroic Spirits having the same Skill, such as the highly universal Battle Continuation.
In Fate/Extra CCC, id_es (イデス, Idesu?) are typically special, inborn Skills possessed by BB's Alter Egos from the moment of their creation. They are "cheat skills" resulting from illegal modification through morphing and reinforcing their existing abilities, making most of them Skills that normal Servants do not possess. They are the exposure of an individual's psyche rather than the abilities of the body and likened to fetishism.
In Fate/Grand Order
The game system of Fate/Grand Order (and subsequently Fate/Grand Order Arcade) classifies skills that are always active as Class Skills and skills that can be activated at any time by the player as Personal Skills. The result is that some inherently Personal Skills are treated as Class Skills, and vice-versa. The English localization labels them "Active" and "Passive" Skills.
Some Noble Phantasms are also treated as Skills, which is occasionally explained as them lacking the power of a True Name release.
Originally, Fate/Grand Order had Servants largely share from a pool of generic Skills, especially in regards to Class Skills, as had been the case with previous Fate works. The few truly unique Skills (固有スキル, Koyū Sukiru?) were indicated as such in character profiles. Later versions of the game began to introduce much more idiosyncratic Skills for each Servant, either upon release or through Strengthening Quests that convert one Skill into another or increase the Rank of a Skill. Furthermore, phrasing regarding the aforementioned "unique Skills" was removed by the time relevant Servant profiles were published in Material books, although they remain unchanged in-game.
List of Skills
|A • B • C • D • E • F • G • H • I • J • K • L • M • N • O • P • Q • R • S • T • U • V • W • X • Y • Z • #|
 Fate/EXTRA material - Encyclopedia: id_es [Skills], p.160 [T]
These are the special skills of the Alter Egos conferred upon them when they were created.
The skills are the result of modifications performed by various hacks. id_es skills are the Alter Egos’ existing abilities that are changed or reinforced to be stronger.
Most of them are skills that normal Servants do not possess.
They are skills that reflect the mind or emotional embodiment of the Alter Ego, rather than being based on their physical ability (body).
The various id_es include Passionlip’s Masochistic Predisposition and Melt Lilith’s Sadistic Predisposition. There are also others like Infantile Regression, A Taste for Bondage, and Cognate Aversion.
 Fate/EXTRA material - Encyclopedia: Violet [Alter Ego], p.160-161
Violet [Alter Ego]
One of the Sakura Five members that didn't make it into the game.
The Alter Ego of purity.
She's the eldest among the quintet, and...... looks....... to be in her mid-20s. She sports long hair and a tight uniform, and has this quiet intellectual feeling to her as the "mature Sakura." Because her role on the Far Side of the Moon is much like that of the Minister of Finance, BB entrusted her control over the world's liquidity resource (mana); she rules over the vault of a megabank. Unlike the other Alter Egos, she doesn't possess quite as unique id_es skills (fetishisms), but that being the case, the level of how sturdy her body is is frighteningly high. She gives more of an impression of being a Servant, rather than an Alter Ego.
Her purity is one that takes the form of affection, and her motif being a desire to monopolize. Loving under a cold exterior, but still taciturn, cruel, and wanting nothing more than to be in complete dominance. She gave herself the simple name "Violet", further proof that compared to the other Alter Egos, she's not interested in playing games. Due to her being the Sakura of purity, her actions and words don't have a wide range of expression, but that's not to say she doesn't hold some interest for our protagonist. In fact, she wants to monopolize him, at the very least. What she seeks is "pure and proper companionship." You could say she's in charge of safeguarding morals on the Far Side.
Her id_es skills are "Shackling Desire" and "Crack Ice/Andromeda." Though she has an urge to chain down both her companion and herself, she does her utmost to keep that urge hidden, but of course, it's already evident to everyone around her. I mean, come on, her weapon's a whip for God's sake! Crack Ice is a pair of mystic eyes that paralyze its target. Even space itself falls victim to these eyes of enchantment. Point is: it stops time. It was planned she'd switch over to the protagonists' side and protect all the masters until the end of the game as a "good Alter Ego."
自身につけた名称が単純に 「ヴァイォレット」 なのも、 他のアルターェゴに比べて遊び心がない証明。