A single point sling is exactly that. One attachment point to your firearm. In most cases there is some form of sling plate, sling swivel stud or stud located on the firearm. In most cases it is toward the rear of the firearms point of balance and if you release the firearm it will drop muzzle down. Works best with short barrel firearms. Its extremely simple to use, however, the drawback is that if not secured with a limb, it will tend to flop all over the place when running/walking/engaging other targets with back-up/secondary firearms.
A two point sling is the more traditional type that most hunters are familiar with. In "tactical" terms, the sling attachment points do not necessarily have to be along the bottom of the firearm, but may also be found on the sides and/or top of the weapon. Again, its a relatively simply design, but with two attachment points, the firearm doesn't tend to flop around as much as with the single point and this type is more suitable to longer barrel firearms.
The three point "tactical" sling is a much more complex affair, involving a strap system that attaches to the firearm at two points, just as with the two point sling, however, an additional strap system is then attached along the length of this two point system via some sort of slider, D-ring or metal/plastic clasp. Through this attachment point, another strap/sling arrangement is run around the shooter's torso and either right/left shoulder, across the back in a diagonal fashion and back up and through the D-Ring/Slider.
Usually there are an arrangement of fastex buckles and sliders that allow each shooter to lengthen or shorten the sling to fit them individually. Some designs employ some form of quick attachment using fastex buckles or similar devices the permit the shooter to quickly detach the firearm from the sling system that is worn on the body in case the firearm needs to be abandoned during combat, loaned to another shooter, or you're simply making a head call and don't want to screw around trying to get out of all the straps, etc...This kind of set-up permits multiple carry angles, both in front and behind the shooter, muzzle up, down or level and comes in handy for guys on guard duty, etc...as the firearm can be kept at low-ready 100% of the time and it gives you some place to rest your hands.

Of the three designs, it probably offers the most secure carry style for your firearm if you have to move on the run as many designs allow you to essentially lock your firearm into one position. This comes in handy when going over obstacles, climbing ladders, etc...
While still popular with some combat troops, a lot of the "high-speed, low-drag" kinda' guys are returning to the more "simple" styles of sling, such as the "Vickers Sling" manufactured by Blue Force Gear & designed by former Delta "operator" Larry Vickers. Vickers and his supports see the 3 point system as being too complex and too easy to become entangled during the confusion of combat. For us range "tacticool operators" it mostly comes down to a decision between what you like best or what feels most comfortable for you.