The distinction between thread and yarn is subtle, largely historical and circumstantial, and both terms are often interchangeable–which can create confusion, especially for a beginner textile worker. Broadly speaking, yarn is for knitting and weaving and thread for sewing and embroidery–though yarn can be used for embroidery and thread for weaving. To all intents and purposes they are the one and same, at least this is how I’ve always understood it and how I use the terms for the purposes of this text.
Thread is made of spun fibres:
Fibres either occur as long filaments – such as silk, pima cotton and all extruded materials such as nylon, polyester etc., or in short lengths – such as wild silk, poorer quality cotton, wool or coconut fibre.
a) a method of joining short, and sometimes long lengths of fibres or filaments to make longer lengths. Hair or vegetal fibres or filaments are always spun–each one overlapping the last to make one long thread. This becomes single ply yarn.
b) a method of joining lengths of fibre to make thicker threads. A single thread might be not strong enough on its own, so two or more threads are twisted together to increase its strength, plyed. This is often referred to as yarn.