Most modern line is made out of a form of plasticiser which covers an inner core.
There are some lines made in a different way but these tend to be non stretch and are often susceptible to horrific memory problems. Lines are normally about 30 yards long and shooting heads about 10 to 12 yards long. The normal format is double taper which, as the name suggests, is tapered at both ends and has a long level middle section. In theory it is possible to turn these round when a section gets worn but in reality the end nearest the backing will be lightly coiled after months or years stored on the reel. The other main type of line is called weight forward.