Guardian
Guardians are your basic Eldar boots on the ground. Every Eldar is given some military training and drill so that a militia can be formed if necessary to defend their craftworld. Guardians are those who are not currently walking the Path of the Warrior, but instead do something like baking or artistic flower arrangement. When the need arises, these civilians are roused and serve as infantry as well as vehicle and starship crews.
Like most Eldar, Guardians are physically weak and fragile, (at least compared to other Xenos and SPESS MAHREENS) but mobile. Unlike most Eldar, they don't have a specialized role—a problem in an army that is all about specialization. They have the luxury of choosing their own role to play but cannot do so very effectively; often it is better to take a unit that can perform a specific role well over the generalist Guardians.
The usual armament for an Eldar Guardian is a Shuriken Catapult. It is assault 2, has bolter strength and AP, and a neat ability called Bladestorm that is basically a weaker version of Rending. Unfortunately these guns have a pathetic range of 12". This is a problem, as most dedicated assault infantry can charge that distance and will destroy guardians in close combat, while most infantry weapons can Rapid Fire at that range, shredding guardians with their low toughness and weak armor. Khaine help you if you face Fire Warriors, who can Rapid Fire from beyond ranges you can even reach. So you want to think carefully before moving Guardians into close range.
Usually you won't be taking Guardians to kill things—you'll want them sitting in cover or on an objective, warding things off with the threat of very killy dakka if they get close. The Eldar have access to very fast, tough jetbikes that can have even more firepower than Guardians for only a marginal increase in cost (why aren't you using them instead?), or Dire Avengers, a type of Aspect Warrior who are basically Guardians++, with heavier armor, a gun with 50% more range, and access to an Exarch with neat powers and a sword that can insta-gib pesky champions or leader units in melee.
On the other hand, Guardians can take a Warlock who has access to a sweet sword and a variety of psychic powers which are mostly good for defense, and can purchase weapon platforms for some long-range firepower (though taking a heavy weapon invalidates the firepower of the rest of the squad due to their short range). The squad size is also two times larger than Dire Avengers and the largest among any in the Eldar codex, useful for when you really need a lot of relatively cheap, short range firepower. Twenty Guardians can lay waste to pretty much anything if they can reach it. Considering all of this, you will have to decide whether it is better to conscript all your citizens into a horde of fragile, short ranged soldiers, or to build your own aspect warrior Swordwind and let the specialized rainbow fly.
It's a mystery to many W40k players that have even a fragment of interest for the lore as to why Guardians still even exist in the board game, considering the Eldar race has been dying out for more than 10,000 years. The oft-repeated saying that "one Eldar life is worth more than a hundred thousand human lives" is being contradicted quite a lot by the fact that the lives of these ordinary Eldar citizens can so easily be thrown away. One would assume that at this point all Eldar would already be walking the Path of the Warrior or Exile or Seer.