Thread count is a measure of quality of fabrics used for bedding. This measure of quality looks at the fineness or coarseness of a fabric; in this case it mostly refers to bed linens like sheets and pillowcases. Thread count is the number of threads going each way in one square inch of fabric.
Thread count is measured by counting the number of horizontal and vertical threads in one square inch of fabric. Thread counts average from lowest (80) to highest (1200). Most are usually between 180 and 300, and anything above 300 is considered higher quality.
Higher thread count means more threads, and this usually translates into a more densely woven fabric that lasts longer and is many times softer. Some exceptions to this rule are fabrics made of linen, flannel, or jersey.
Fabrics like linen, flannel, or jersey are subject to low thread count because of the type of material they are made of and the properties of that material. For example, part of the softness of flannel comes from its low thread count because of the material its made from.
Ply and thickness are two factors that also affect thread count. The ply is defined as the number of threads that are wrapped into a single thread. Single-ply refers to the use of single threads in the construction of a fabric. Two-ply refers to materials that twist two threads together into a single thread, thus increasing the fabrics thread count and creating a stronger thread. This creates a heavier, more durable fabric.
Using finer threads often allows for softer, smoother fabrics and more fit into a square inch. Finer thread, like silk, create a lighter fabric, and are thus more desire able than coarser threads. They are also more fragile, so it is important to care for them especially well.
While thread count is important in picking your bed linens and sheet sets, remember that cotton and silk are the main types of materials to judge based on a high or low thread count. Other materials like linen, flannel, and jersey may have a lower thread count, but this doesn't mean they aren't comfortable. Blends are also common, and often create softer materials. Jersey-cotton blends, for example, are ideal for baby sheets and other types of baby bedding.
When in doubt, see what feels the best. You're best off visiting a fabric store and exploring the many different kinds of materials to find out how a fabric feels. Knowing what you're going to get makes your sheet shopping experience easier and more rewarding, especially if you're shopping online.