It is strong, holds up well to the weather and stress, and coils nicely without too much kinking. Nylon three strand is the preferred line for docklines, since it stretches sufficiently to dampen the sharp shocks of wave action and wind against your cleats. Dacron doesn't stretch as much, and is used for sailboat running rigging and other applications where you don't want stretch to interfere with your sets. The big advantage of polypropylene line is that it floats. Therefore, it is appropriate for ski lines or other applications where you want to be able to see the line on top of the water.
Line is constructed in two basic ways, although there are variations on the theme.
If you hold a length of 3 strand right-hand laid twisted line at arm's length and eyeball it, you will see the wrap of the line twisting to the right. The other construction type is braided line. Braided line can be single or double braided, and in both cases the line is braided around a central core. This type of line does not stretch to the degree that twisted line does, and is more difficult to splice. However, it goes through a pulley or block very well because of it's rounded shape, and is stronger than its equivalent size twisted line.