Silk is the finely woven thread of the 'silk worm' - a caterpillar (Bombyx mori is the most common species, but there are others) that produces silk while building its cocoon in preparation for pupation.
The thread is taken from the cocoon before the moth fully passes through the chrysalis phase. This process kills the moth, so obviously not every chrysalis is taken.
The colour of the silk depends on what the caterpillars were fed. They require green leaves, and are generally fed on mulberry leaves which give a very pale blonde or white, depending on species. If the diet is supplemented with rose petals, the thread can come out with a pink tinge.
High-quality silk is reeled rather than spun; the individual fibers are lifted out of a hot water bath to form threads. Generally only the waste silk from broken cocoons is spun.
Velvet was made from silk.