Rangers are the sniping and sneaky experts of the Eldar forces in Warhammer 40k. They are exiles - either self-imposed or forced - from their craftworlds because they can't stand the boring Paths of the Eldar and want to see the galaxy, explore it a bit and have a life. While this may seem quite sensible and you may sympathize with them, the downside is without the mental constraints of the Paths, the Eldar exiles are free to be all nutty like their ancestors were when Slaanesh was born and nommed them, so the drawback of possibly being daemon food may make you think twice before picking this kind of life.
Anyway, Rangers are still loyal to their craftworlds and can be called upon to assist in times of need. They do this by using Ranger Long Rifles and taking out various key threats before they can reach the Eldar on the battlefield. Since they come in squads, the prospect of a bunch of invisible (they have powerful camo cloaks) Eldar snipers is not a pleasant prospect.
They are often used as a character class if the Eldar are included in a 40k RPG as they can make interesting characters with their wide spread travels allowing them to build up a range of skills, abilities, and meet various potential allies and foes.
In Dawn of War, they were annoying for their invisibility and sniping fire, especially since they also took away morale like no one's business. Unless you had a hero or a sensor it was very difficult to counter them....grrr unless you got your boys throwing grenades in their general direction. Take that you tree hugging bastards...
On the TabletopEdit
8th edition Rangers are arguably the most defensive troop choice Craftworlders can field, and are the second cheapest to boot. Not only do they gain additional protection while in cover (effectively a 3+ save), but they also have a native -1 to hit modifier that can be stacked all the way to a -3 in emergencies (-4 for Alaitoc Rangers). If those reasons weren't convincing enough for the skeptical players, Rangers are also the only non-Warlord Aeldari units that can target Characters freely, making them perfect for taking down enemy Apothecaries, Warlocks, PainBoyz and the like.
They do have their downsides, however. Rangers only have single shot rifles with mediocre strength and no AP to speak of, shuriken pistols and no access whatsoever to any heavier guns. This basically means that they will struggle against hordes, vehicles and monsters (though they can still do surprising amounts of chip damage with Mortal Wounds). Also, as their rifles are considered Heavy-class, they are one of the few Aeldari that suffer for any movement at all (particularly unheard of for Craftworlder infantry).
All in all, they are still fantastic troop choices (particularly in Alaitoc lists, which offer additional bonuses to them in particular) that make excellent mid-to-backline objective holders and can be strategically deployed to help influence or punish enemy character placements.