Sniping is in my opinion, an art. Picking off people at long ranges is quite fun. It takes quite a bit of skill too, which I will cover in this section.
Personally, I'm not an experienced sniper myself, but seeing as no one here has begun to write on sniping, I thought I should.
Basically in my mind, there are two main forms of sniping: Camp-sniping and Mobile-sniping. Every sniper rifle has it's special uses and functions.
The Sniper Rifles are:
The M24 Weapons System
The M82 Semi-Automatic Anti-Material Rifle
The Special Purpose Rifle
The Mosin-Nagant Sniper Rifle
The SVD Sniper Rifle
The VSS Vintorez
Additionally, anything from an RPG-7 to an M9 can be used in sniping(I picked off someone on Middle Tower Top in Bridge Crossing with an M9 while in Defense Tower once), it's just that the aforementioned are better at what they do. I'll be covering this as well.
General Tips
Generally, sniping is best used in long range battles, not CQB. Though the Vintorez has a full auto function, it's meagre ten-shot magazine and lack of an unzoomed crosshair makes it inferior to the M9 Berreta in terms of CQB. However, If you're planning an ambush in very cramped spaces, you might consider a one-hit kill weapon like the M82.
Moving on, Camp-Sniping is what I call it when someone sits in a fixed place for a long period of time(with bipod down if he has one), camping and waiting for someone to pass by. This is actually quite safe, as long as you don't slack off while watching your scope, or someone shoots you from somewhere outside your area of vision, like behind you. The upside to not moving at all will be that people from far away might think you're just some staionary shrub or structure and forget about you. The downside is when someone has seen you. If you stay in that one position, he'll know where to shoot at every time, whereas he keeps moving.
Mobile Sniping is defined as moving from place to place, setting up the rifle and possible the bipod for a short period of time before moving on. In this way you won't be able to watch one single area at 100% effectiveness, and yes, it increases your visibility, with you scurrying around everywhere. However, if someone sees you, a Mobile Sniper will simply move on to somewhere else and the enemy will have to look for you again.
The M24 Weapons System
The M24 Sniper is an ideal weapon for sniping at long ranges, as preivous beneficiaries of it's low breathing problems and high accuracy can testify. The M24 can be fired with great accuracy in prone or crouch postion without a bipod, and experienced players can even fire while standing. A shot in the chest or head is usually an instant kill, but a shot in the hands or legs will only slow enemies down. This is a good rifle for beginner snipers. More information can be found on the M24's page.
The Mosin-Nagant Sniper
The Mosin is the OPFOR equivalent of the M24, and the breathing shouldn't bother you too much. It has no bipod and is better suited for medium-range sniping as it does not have a very powerful scope. More information can be found on the Mosin's page.
The M82 SAAM Rifle
The M82 is one of the most used rifles in the game. It's high-kill ratio comes mostly from the fact that a single shot in the chest is enough to kill. The M82's power is accompanied by some smoke and a bang when it is fired. Also, you will be forced out of your scope after every shot. Breathing problems make this rifle most suitable to be used by an experienced sniper. It is good for camping, as the bipod will negate the breathing problem and the 'one shot one kill' policy of the m82 is good for this purpose. It is not recommended you use this rifle while standing or moving. It is also not a bad tool for rapid sniping(taking quick shots in fast sucession) as it can shoot eleven times before having to reload. More information can be found on the M82's page.
The SVD Sniper Rifle
The SVD is the OPFOR equivalent of the M82. It comes with less recoil, but a less powerful scope and no bipod. Ideal for medium range engagements. More info on the SVD's page.
The SPR
The Special Purpose Rifle is essentially a modified M4A1 with a strong scope and a cute 20 shot magazine. It is also equipped with a silencer, and a bipod. This rifle will do little damage, so shots in the chest and other areas will require extra hits to kill. However, due to it's low recoil, high accuracy, strong scope, and a 20-shot mag, you should always aim for the head when holding this gun. It is much easier to pull off a headshot using the SPR. More info on the SPR's page.