Thread count refers to the number of threads, both vertical and horizontal, in a one-inch square of fabric. It is affected by a number of factors, including ply and thickness of the threads used. The ply of the fabric refers to how many threads are wrapped together into a single thread. Single-ply fabrics use threads on their own, while two-ply fabrics twist two pieces together into a stronger thread, as well as doubling the thread count of the fabric.
Using finer threads also allows for more thread to fit in a square inch. Finer thread often results in smoother, softer fabrics, part of the reason high thread count fabrics are considered more desirable than fabrics without one. Finer thread also results in a more fragile fabric, however, which may not always be ideal. Two-ply fabrics help solve this problem somewhat by strengthening the threads and creating a more durable, though heavier, fabric.
While it has become common to shop for such things as bed linens based exclusively on thread count, it is important to take other considerations into account. How the cotton is treated can be a much more decisive factor in comfort and overall feel, as can the final finishing of the fabric.
The general wisdom is that a thread count of higher than 100 is desirable --these fabrics are known as percale -- with somewhere above 180 being ideal. While fabrics are available with counts up to 1000, anything in excess of 400 is considered by most to be simply extraneous. In the case of some fabrics, it is simply not a viable option.
The most common products for which this aspect is an issue are sheets, and more and more linen manufacturers sell their products largely on the basis of having a high thread count, as well as the inclusion of high-status materials such as Egyptian cotton. Other products are jumping into the thread count craze, however, with everything from dresses to men's shirts listing thread count on their packaging.