THE SNIPER
The sniper's primary weapon is, of course, his sniper rifle. Using a laser sight to accurately place his shot, a sniper can take out opponents with a single bullet.
The rifle can also be used to as an auto-rifle (like a machine gun). These rounds do less damage than in sniper mode, and works best during close quarters combat.
The sniper also carries the nail-gun used by other classes.
As with all classes use the crowbar as a last resort.
Special Skills
Hitting the special skill key (right mouse button by default) will give you the sniper rifle's zoom mode. Here you have a zoomed in look to line-up your unsuspecting targets.
Grenades
The sniper can only use the fragmentation hand grenade. The sniper is the only class that carries only one type of grenade.
Strengths
Without a doubt, the biggest strength of a sniper is to use his zoomed in sniper rifle and take out enemies with a single shot to the head. While this sounds easy, no one just becomes a good sniper on their first day. Successful snipers will need patience and accuracy. The first key is to find a good sniping perch. Look for somewhere high. Make sure this place overlooks a well traveled area and that you can look a fair distance away. Remember, you can harm them well before they even see you. Once you have found your perch, switch to the sniper rifle and zoom in. Hold the primary fire button down and the red dot should appear. The longer you hold down the button, the more damage your shot will do. After three seconds, the shot will be fully charged. A good sniper will hide his dot so that his enemies won't see it until it's too late. Opponents will be on the look for the tell-tale sign of a red dot. Hide your dot behind walls, on columns, or even into the water. When the bad guys appear, aim for the head (this will do the most damage) and let the button go. Note that a shot to the leg will halve their foot speed.
Another key to being a good sniper is stealth and surprise. The first time an opponent should hear the sharp report of your rifle is when the bullet is almost into his head. Sometimes, you've charged up your shot only to find you want to move your perch. Letting the shot go will let the enemy know that a sniper is around and where you are potentially. Keep the element of surprise by jumping in the air and letting go of the button. A message will pop up about only sniping with two feet on the ground and your shot won't go off. Now high-tail it to where you can get a good shot off!
If enemies have gotten to your snipers perch, and a close-in battle is imminent, switch from your sniper's rifle to the auto-rifle. Some really good snipers will try last ditch snap shots with the sniper's rifle (they don't even bother to zoom). This is very risky and best left for experienced players. Instead switch to the auto-rifle and use your grenades.
Snipers seem to be equally effective for attacking and defending. As defending goes, get the sniper to a high spot and cover an entrance. The enemy will have a hard time not losing men to the deadly red sight of the sniper. As attacking goes, the sniper is more effective than most people think. Sometimes, a sniper is a good idea to bring as backup. Move him into position and give cover to a comrade running with the flag. The enemy will never know what hit 'em!
Weaknesses
Strangely enough, I believe the sniper's biggest weakness is ping latency. Even with the newer net code, you still have to be ready for lag spikes. This is caused by several factors and you have to adjust when sniping. Try aiming slightly ahead of your target when you shoot. People with low ping (LPB - low ping bastards as they are called) don't have to worry about this as much.
Another weakness the sniper has is his extreme vulnerability when zoomed in close to a far away spot. At this point, the sniper has no idea what is happening close around him. Any opponent can sneak up to him and get some in close shots before the sniper can react. The sniper's mortal enemy is the spy. A spy can sneak up behind a zoomed in sniper and kill him with one stab to the back. When playing sniper don't stay zoomed in for more than 20 seconds. If no targets present themselves in that time, zoom out and assess the safety of your sniping position. Also, don't snipe from the same position everytime. The other team will know where you are and find you. I recommend changing sniping positions every two kills.
Bottom line as a sniper... be paranoid.
How To Kill A Sniper
Keeping the enemy far away gives the sniper a huge advantage. Remove this advantage by quickly closing the distance between you and him. The best way is to use stealth, so that the sniper doesn't even know you are coming. If he does see you coming, make it difficult for him to get a good shot off. Jump up and down, strafe left and right, and don't run in a straight line toward the sniper. If possible, fire at the sniper while closing the distance and toss a grenade at him.