"At close quarters, an archer is an unarmed man."
-Sun Tzu
Marksmanship is a Hunter talent tree. Marks used to be the dominant Hunter spec before the release of the Burning Crusade expansion. Patch 2.1, with its improvements to Survival, as well as The Burning Crusade's review of Beast Mastery, has knocked Marksmanship off its throne a bit, but 20 points in Marks (down to and including Mortal Shots) is still largely considered mandatory for Hunters, irrespective of whatever other tree the Hunter chooses to invest the rest of their points in. Additionally, the tree is still enormously viable in many scenarios, particularly PvP, due to the high burst damage provided not only by Lethal/Mortal Shots, but also by the attack power enhancement talents deeper in the tree.
The tree's main purpose is to deal major amounts of ranged damage. It does so by several talents that increase the overall damage of the hunter—such as the Ranged Weapon specialization, 
[Lethal Shots] and 
[Mortal Shots] talents, and bonuses to specific damage skills—such as 
[Barrage] and 
[Improved Stings] and also offers an additional high-yield skill, the 
[Aimed Shot].
The pinnacle of this talent tree used to be 
[Trueshot Aura], which at the highest level grants 125 additional attack power to all members of the group the Hunter is in, making the hunter very useful in groups of other hunters, rogues, and warriors. The 41 point Marksmanship talent is Silencing Shot, which prevents a target from casting spells for 3 seconds. This is effective against casting player characters (warlocks or shamans) in PvP as well as its excellent application for PvE crowd control (pulling casters into the hunters freezing trap in instances and raids.) And the current 51 point talent is Chimera shot which deals 125% weapon damage, refreshes the sting on the target and triggers an effect based on whatever sting is on the target.
With recent patches and the implementation of the Ulduar raid instance the markmanship tree has enjoyed a resurgence among hunters in raiding guilds in much the same way that Wrath of the Lich King made improvements to the survival tree. The mana-inefficiency noted as the primary downfall of hunters specializing in markmanship has largely been addressed as of patch 3.1, particularly so in raiding environments where mana users benefit from multiple sources of 
[Replenishment]. Both the survival and markmanship tree currently produce high dps, with the markmanship tree possessing slightly higher dps potential than survival when the highest level of gear is available.
For detailed info, see Marksmanship section of Hunter talents.