Two point slings
The most classic and popular type of slings. Placing on weapon in two places, reliable fix it. Highly popular in forest, patrols and almost everywhere, when you don't need quick access to weapons.
One Point Slings
It's main plus - they are provide fully control under gun, do not restrict movement. They are light and cheap. Highly popular in CQB and for it's main aim they were created.
Three point slings
There most innovative type of slings. It's something middle between one point and two point sling. Provide quick access to the weapon, but in the same type you can easily switch to the patrol mode. Main minus of this slings - difficult installation on weapon and high price.
Don't want to buy one more time your favorite Glock 17? Don't forget to buy pistol strap! Avoid miss of your secondary weapon, even if it flies out of the holster.