Here are the full details on Servants. Note that they are considered phenomena.
Heroic Spirits
ëpùý - Eirei
A phenomenon and type of elemental. Basically, the elemental force of humanity.
A word that had had also been used to designate those heroes who died on the battlefield. In other words, dead soldiers. But in this case, better thought simply as dead heroes.
Those who accomplished great deeds during life and became objects of reverence and lore after death. The greatest entities that exist as a force "that protects humanity" and are categorically closer to elementals than wraiths. While elementals are a power that uses "images" created by humans to serve as a container to materialize, what's inside that container for Heroic Spirits is also strung together by the hopes and wishes of mankind. Among the Heroic Spirits, there are those of legends, those that actually existed, and those that remain unknown.
Of course, since there are so many crises going on in the modern era and being solved in the blink of an eye, nowadays nobody is really fit in the grand body of humanity to be called a "hero", at least in respects to those from the past.
Normally, Heroic Spirits are summoned as pure power. Receiving a form and a mind similar to what they were back when they were alive is limited only to the Fuyuki Grail War. Generally when Heroic Spirits are summoned, they're used in Kuchiyose or oracles (as in power is channeled through a medium who then makes prophetic statements or what not).
From the 3rd Heaven's Feel, spirits that aren't technically Heroic Spirits (such as the 5th Heaven's Feel's Rider and Caster) have been summoned. As long as the spirit has a side that is also like a Heroic Spirit, they are summoned, with the obvious limitations of the Greater Holy Grail.
Basic powers are not sufficient to contend against powerful thaumaturgy and mysteries, but their Noble Phantasms can allow them to even destroy higher ranking spirits. While having attributes that are similar to elementals due to divinity, worship (wishes), or contracts with the world, they are/were fundamentally human. A Heroic Spirit will not become a god just by worship, however. Normally exist as neutral personality-less power that can be tapped into. Located in the Throne of Heroes, a different plane of existence separated from the cycle of transmigration (i.e. time). Once incorporated into the world system, they are impossible to erase/destroy, unless an error/paradox occurs that becomes too complicated for the world to support. And even so, this doesn't guarantee that they'll cease to exist; they will continue to exist, no matter what happens.
Copies/emanations/projections are sent out from the Throne of Heroes across all times. What is gained and learned from the emanation is not carried on over to the Heroic Spirit to prevent any paradoxes in what ought to be a completed/perfected/finished being. However, the record of what happened is returned to (or rather, has always been in) the Throne, as well as the power that was sent out to solve the problem. This applies to all Heroic Spirits, not just Archer.
Arthur's soul is still alive and not in the Throne yet as she is not a Heroic Spirit. Which is why she carries on the direct memories instead of a record of what she did each time until she obtains a Holy Grail. In her case, she is more or less frozen in her time and what she experiences each time is like a dream to the body in her time line. As she's binded to that, she's still considered "dead" in the modern world, meaning she won't age, can't take surgery, etc., just like other Servants. Restoration of the soul itself (as opposed to bringing a "dead" person back to life through the use of time travel, parallel worlds, and denial of nothingness; the result is still the same but the process and operating principles are different) as a being that's part of the current world time line is only possible through the completed materialization of the soul.
While modern weapons are powerful, those people who use modern weapons mainly are somewhat difficult to be put in the Heroic Spirit category. The greatest advantage of modern weapons is that "anyone can use them with the right training". But see, the thing behind that is that because of that, it's hard for them to become "one sole existence". These are what people call faceless heroes. If there is a hero that has on him modern weapons, what's going to be treated as the Heroic Spirit isn't the owner, an expert that anyone can become by training, but the "most excellent weapon of that era" itself. The weapon itself would have to have a soul however.
Counter Guardians
ù}Ä~é?ÄþîýÄÊ - Yokushi no Shugosha
Guardians.
Heroic spirits that are used by the world to prevent the destruction of humanity but only on the condition that those disasters had been caused by humans themselves. In other words, they are exterminators, summoned not to save lives but to take lives to guarantee the survival of the greater whole of the human species. As long as humans exist, they'll always keep running toward self-destruction, at least in the eyes of the overall group will to survive. It doesn't matter if the intentions are evil or not or if the potential benefits are good; the human counter-force will react to it, crushing it as long as it perceives it to be the cause of ruin for humanity. That's the basic nature of the system - group happiness instead of individual happiness. As Archer learned in death, the justice he was looking for was ultimately just one of the strong feeding off of the weak and then discarding them if they end up threatening the world mankind lives in.
If worshiped or divine Heroic Spirits are natural phenomenon that just happen to be beneficial to humanity, then Counter Guardians are a strictly human phenomenon.
Generally summoned as mindless "power" and only after a disaster has happened with the intent of containing it. Cutting off an arm to save the rest of the body is the operating principle. Whether or not they take a form is unknown. According to Archer, becoming a Counter Guardian is the same thing as becoming a force of "nothing". Sent out by the world, but a guardian of humanity. Divinity or worship can keep one from being allocated into the Counter Guardian slot. The only Heroic Spirit that's a Counter Guardian in Fate is Archer.
Seven of these are used to control Primate Murder.
Some hinting that only Counter Guardians can be summoned "backwards" in time, probably because they're nameless (or are just a representation of a primitive or already innate defensive mechanism).
Reverse Heroes
ö¢ëpùY - Haneiyuu
Those who are hated by people, whose evil actions save people as a result, who clarifies good through evil. While being cursed, they are saviors that became worshiped. Of course, pure Reverse Heroes don't exist. At best, they reach the stage of being those who did good while being evil. Like heroes, they have served mankind when they were alive. However, it was neither the intended result nor were they ever appreciated for it. A target of continuous human hatred and disgust. Generally speaking, unwilling sacrifices, those that were viewed as evil but the end result whether by victimization or actual results saved the world. Of course, there is no such thing as a pure reverse hero, because as time goes on, that hatred and disgust is replaced with feelings of appreciation or guilt. Technically speaking, Archer is both a Counter Guardian and a Reverse Hero. Assassin and Archer fall in this category. Rider, Caster aren't really normal Heroic Spirits but that doesn't mean they are Reverse Heroes.
In addition.
That which is born by humanity, but develop without its help. This contradiction is what results in the creation of a being that is exempted from the pressure of all Counter Forces, an "enemy of the world",
