The term sniper supposedly got its roots from 18th century British Army officers, who tested their marksmanship skills on the small birds called snipes.
Just as important as marksmanship, a good sniper must have superb patience and observational skills. In the book Inside Delta Force, by CSM Eric L. Haney (ret.), Sergeant Major Haney goes in-depth into the things that he learned while in the Operator Training Course.
"We do more observing and reporting than we do shooting- so perhaps we would be better called 'observer-reporters' than 'snipers'."
usmilitary.about.com - The sniper has special abilities, training, and equipment. His job is to deliver discriminatory, highly accurate rifle fire against enemy targets that cannot be engaged successfully by the regular rifleman because of range, size, location, fleeting nature, or visibility. Sniping requires the development of basic infantry skills to a high degree of perfection. A sniper's training incorporates a wide variety of subjects designed to increase his value as a force multiplier and to ensure his survival on the Battlefield. The art of sniping requires learning and repetitiously practicing these skills until mastered. A sniper must be highly trained in long range rifle marksmanship and field craft skills to ensure maximum effective engagements with minimum risk.
The fundamentals of sniper tactics, besides marksmanship, is observation. Snipers, skilled in stealth, are often employed to serve in an overwatch position during most military operations.
A Sergeant First Class I met at Fort Bragg was a sniper in his Cavalry company in Iraq. They recieved reports that local militants were training would-be insurgents on how to operate various weapons in a field in the city. "Jake the Snake" and his observer crept up near the field, into an abandoned building adjacent to the field, a few hundred meters away. Meanwhile, a fire team took up positions perpendicular to his line of sight, outside the field. He watched for hours in the blistering desert heat, waiting for the insurgents to show up. Just as he and his observer were about to pack up, Jake saw the men walking onto the field. A fairly decent sized group formed, and the men talked and laughed completely unaware that a few hundred meters away a man stood watching their every facial expression. You see, over this course of time, a sniper will become very close with his target. He can see everything through the scope of the weapon. A few minutes later, the group forms up, and the leader begins talking to them. As he's talking, he picks up what appeared to be an RPG-7. Jake took aim center mass, and fired. By the time he chambered the next round, and got back down on his scope, the 240B fire team had opened fire and was already converging on the group of former insurgents. Not one body stirred. "They were stone dead, not even moving. And it only took me a second or two to chamber another round, and by that time the 240-team was already on their way to check the bodies."
Also, as equally important, is what type of rifle and ammunition you use. Each different round has a different ranges and velocities, meaning where one round might penetrate a person's skull at 300m, a different round might penetrate clean through, and through the person behind him as well.
Patience
An sniper-observer team may lie in wait for days, waiting on targets to enter his area. The team has to deal with drowsiness and fatigue, and not just of the body. The eyes, in particular, will become fatigued after looking through a scope for so long. In addition, certain techniques are taught to sniper-observers to deal with discomfort. SGM Haney claimed he was taught how to warm independent fingers or toes while laying still, just by concentrating.
Psychological Factors
A few of the problems that may arise. Texas Tower Syndrome, or the overwhelming feeling of power that one might get from taking a life so decisively. Many snipers have claimed to feel this little tug at the back of their mind. The other is called Munich Massacre Syndrome - or the inability to take said target's life once the sniper-observer has become so personal with his target. (Inside Delta Force, 115)
In short, being a sniper is not all glamour and glory and honor. It takes a certain type of person to come out on top of the mess. "Jake" was the kind of guy that could kill a man and not think twice about it, but really a nice guy. SGM Haney often speaks of his regrets at what he had to do, but I doubt he would take it back.
The US Army Sniper School, at Fort Benning, trains soldiers on the use of the M24 Sniper Weapon System. Students must be able to hit targets consistently at 600m. Training also includes the .50 caliber Barret Anti-Materiel Sniper Rifle. Students must be able to observe small targets hung in trees at great distances, and one test requires the student to find a single object in a field through his scope.
The sniper's motto - One Shot, One Kill