There are six main types of gun slings, each with varying degrees of popularity. As they have an impact on carrying and firing, it’s worth considering all of them.
A single-point sling is the kind you'll usually see on AR15s and similar weapons. It fixes to a central point on the firearm, which rests across your chest. This allows you to move quickly to a firing position. The disadvantage is that you usually need one hand on the weapon to stop it from flapping around, particularly if you're moving quickly.
The two-point sling is probably the most common, attaching to the stock and a forward position. Its main benefit is easy, hands-free carry. There is a similar model called a gun sling with an extra central fixing point that allows for the "hasty" position (see below). Many firearms don't have the required attachment point and the gun sling itself is perhaps a little more complicated than is necessary.
The three-point sling is another front-carry model with an additional strap to allow hands-free movement. It's effective, but somewhat complicated and tangles are not unknown.