William K. Purvez in Scientific American:
Spider silk is not a single, unique material–different species produce various kinds of silk. Some possess as many as seven distinct kinds of glands, each of which produces a different silk.
Why so many kinds of silk? Each kind plays particular roles. All spiders make so-called dragline silk that functions in part as a lifeline, enabling the creatures to hang from ceilings. And it serves as a constant connection to the web, facilitating quick escapes from danger. Dragline silk also forms the radial spokes of the web; bridgeline silk is the first strand, by which the web hangs from its support; yet another silk forms the great spiral.
The different silks have unique physical properties such as strength, toughness and elasticity, but all are very strong compared to other natural and synthetic materials. Dragline silk combines toughness and strength to an extraordinary degree. A dragline strand is several times stronger than steel, on a weight-for-weight basis, but a spider’s dragline is only about one-tenth the diameter of a human hair. The movie Spider-Man drastically underestimates the strength of silk—real dragline silk would not need to be nearly as thick as the strands deployed by our web-swinging hero in the movie.