Thread count is simply the number of threads per square inch of fabric. These consist of vertical threads (warp) and horizontal threads (weft) woven together. Construction refers to how the thread count is achieved (# of warp and weft yarns, # of picks in the weft, use of 2 ply yarns etc.) To achieve higher thread counts, sometimes 2-ply yarns are used and sometimes multiple yarns (picks) are inserted into the weft.
The FTC has ruled that plied yarns should each only be counted as one thread for the purposes of thread count. This is not enforced, but in response the market has moved more toward single plies with multiple picks as the preferred method of achieving higher thread counts. In weave quality terms alone, the best fabric would be made with single ply yarns and have a single pick; but the highest thread count you can get with this type of construction is about 400. Above that, 2 ply yarns and/or multi-picks must be used.
The buzz about "single ply" in the last five years or so, was a reaction to customers feeling cheated by the concept of 2 ply. (meaning a 300-thread count construction made with 2 ply yarns and called a 600 thread count) But the "single ply" concept has its own problems, as stated above. Sheets made with "single ply" yarns but with 6 to 8 picks do not necessarily result in the best feeling or highest quality weave – – but they do achieve the higher thread count in a way deemed more correct by international standards and the FTC.
In a quality product, the incremental comfort value of thread counts over 300 is very little. A 300 thread count can feel far superior to a 1000 thread count. Thread count has become a simple measurement used by marketing people to capture interest and impress with high numbers. The problem with mass produced high thread count sheets is that to keep the price down, important elements of quality must be sacrificed, meaning in the end the customer gets a product with an impressive thread count but that probably feels no better (or even worse) than something with a lower thread count.
How does this happen?
Weaving with 2 ply yarns that do not have a high enough yarn size so the end product feels heavy and blanket-like.
Inserting multiple yarn threads (picks) into the weft. These are often visible to the naked eye. I've heard of as many as 8. This practice increases the thread count but otherwise really has no practical or useful purpose. Depending on the number of picks and yarn size used it can also make the product feel heavy.
There is no simple answer to the thread count, ply and pick game; there are thousands of combinations that will make a beautiful product. I see many excellent examples of every type of construction in the marketplace (thanks to quality fiber, yarn size and finishing). Keep in mind that with higher thread counts, price and quality do tend to go hand in hand. An extremely high thread count sheet at a very low price is exactly what it sounds like: too good to be true. This is not to say that you have to spend a small fortune for quality sheets – just don't fall into the thread count trap. Unfortunately, a lot of companies don't make it easy to be well informed.
More things to consider:
Fiber Quality - www.rosssveback.com/2009/08/linens-fiber-quality.html