Yarn is a long continuous length.

Yarn is an extended unremitting length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and rope making. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or other lubricants to hold up the stresses involved in sewing. Embroidery threads are yarns specifically designed for hand or machine embroidery. Spun yarn is made by twisting or otherwise bonding staple fibres together to make a cohesive thread, or "single." Twisting fibres into yarn in the process called spinning can be old-fashioned back to the Upper Paleolithic, and yarn spinning was one of the very first processes to be developed. Spun yarns may contain a single type of fibre, or be a blend of various types. Combining synthetic fibres (which can have high strength, luster, and fire retardant qualities) with natural fibres (which have good water absorbency and skin encouraging qualities) is very common. The most widely used blends are cotton-polyester and wool-acrylic fibre blends. Blends of different natural fibres are common too, especially with more expensive fibres such as alpaca, angora and cashmere. Bamboo yarn is a less luxurious kind so as to is a recent modernization.

Yarns are selected for different textiles based on the characteristics of the yarn fibres, such as warmth (wool), light weight (cotton or bamboo), stability (nylon is added to sock yarn, for example), or softness (cashmere, alpaca). Acrylic yarn is the least expensive.

Yarns are made up of a number of singles, which are known as plies when grouped together. These singles of yarn are twisted together (plied) in the opposite direction to make a thicker yarn. Depending on the direction of this final twist, the yarn will be known as s-twist or z-twist. For a single, the direction of the final twist is the same as its original twist.

Filament yarn consists of thread fibres (very long continuous fibres) either twisted together or only grouped together. Thicker monofilaments are typically used for manufacturing purposes rather than fabric production or decoration. Silk is a natural filament, and synthetic filament yarns are used to produce silk-like effects.

Texturized yarns are made by a process of air texturizing (sometimes referred to as taslanizing), which combines multiple filament yarns into a yarn with some of the characteristics of spun yarns.