Archers are a Class of Unit (as in "archer-class infantry").Erf-b1-p077a. An archer uses projectile weapons, such as a bow and arrows or crossbow,Erf-b1-p049 Erf-b1-p059 as its weapon. Any unit can, theoretically, throw or fire a projectile weapon. However, the archery specialization gives a unit a much better chance of hitting its target, while a non-archery proficient unit's chances of hitting the target are greatly reduced.  Archery works independently of Engagement.(according to LIAB Text 52)
Archer-type units have the ability to strike at flying units.
WoodsyErf-b1-p057 and Shady Elves, as well as Men, all have archers. While men have other unit types, it is possible (though unclear) that Woodsy and/or Shady Elves only have archer units, or that all such elves are automatically archers, in addition to or instead of, any other classification. It is likely, though not supported directly via the comic, that Shady and especially Woodsy elves are superior archers to all others. (As this is a common feature of such units in other games).
The maximum range of Archers is unknown. It's probable that they can reach any unobstructed target within their hex. It might be possible for them to hit units in other hexes, although if that's the case there may be other restrictions like only firing on their own turn, perhaps as a trade-off for movement.
Ansom, in Love is a Battlefield, Page 9, orders his flying units, which are under archer fire, to land on the ground. This implies that archery has some sort of bonus against flying units, and that landing on the ground would negate that bonus. This is supported by the fact that in Warlords III, which Erfworld draws inspiration from, all units with the archery special ability gain a +2 bonus against flying units.