Slings are a critical piece of equipment for the rifle. They are like a holster for your long gun. Anyone that has gone out without a sling can tell you that carrying the rifle by hand creates problems. The lack of both hands to balance, climb or do simple tasks can be troublesome. Also, carrying a rifle by hand can be more fatiguing than using a sling, leading to decreased performance when the rifle is needed.
Hunting slings are relatively unchanged from the old straps of the frontier. They are often a straight strap used to simply hold the firearm on the shoulder so it does not have to be carried by hand. They can be found in nylon and leather, sometimes with quick adjustment so you can use the sling as support for an offhand shot.
The safari sling is gaining popularity as well. Originally intended for big game hunters in Africa, the safari sling offers an adjustable fit loop for your elbow (photo). This type of sling is usually more expensive than standard options, but is extremely useful for taking standing shots.
Tactical slings are significantly more varied than what is offered in the hunting market. Generally they can be broken down into three categories: single point, two-point and three-point slings. All have their own benefits and detractions.
The single point slings attach to the rifle at one point at the rear. They are great for range use and vehicle operations (if you are in and out of a vehicle constantly). It does not take long, however, to find out how much the rifle moves when you have to walk longer distances. They work well for static positions, but not as well on the move.
Two-point slings are more versatile when they are carried properly. Two point slings can be carried across the chest with the muzzle down, which keeps the gun at the ready. Ideally, this style sling will come with a quick adjust for length to you can tighten the rifle to your body when you need to move quickly or loosen it quickly to employ the rifle. It can also allow you to support an standing shot like a hunting sling.
Three point slings are useful, but use quite a bit more strapping. This means the three point sling takes longer to learn and may have more potential to snag on other equipment. The advantage to the sling is that the rifle can seamlessly go from ready carry in the front to slung over the back in seconds. This makes it simple to perform two handed tasks without the bulk of the rifle on your chest.