Yarn 
Yarn is a long continuous length of interlocked fibers, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving and ropemaking. Yarn can be made from any number of synthetic or natural fibers. more...
Yarn can be made from a variety of natural fibers including wool, alpaca, angora, cotton, silk, linen, bamboo, hemp, and soy. Less commonly, yarn is spun from camel, yak, possum, cat, dog, wolf, rabbit, buffalo hair and even turkey feathers. Commercial yarns are often made from synthetic fibers or a combination of natural and synthetic fibers. Very thin yarn is referred to as thread. Yarns are made up of any number of plies, each ply being a single spun yarn. These single plys of yarn are twisted in the opposite direction (plied) together to make a thicker yarn.
In some cases, thread may be monofilament, in which case it is a single fiber. The only natural fiber that is counted as monofilament is silk.
Yarn is manufactured by either a spinning or air texturizing (commonly referred to as taslanizing) process. Yarn manufacturing was one of the very first processes that was industrialized.
Yarn used for fabric manufacture is made by spinning short lengths of various types of fibers. Synthetic fibers which have high strength, artificial lusture, and fire retardant qualities are blended with natural fibers which have good water absorbance and skin comforting qualities, in different proportions to manufacture yarn for fabric. The most widely used blends are cotton-polyester and wool-acrylic fiber blends.
Read more at Wikipedia.org