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. 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 thus resulting in exaggerated thread count.
Don’t fall into the thread count trap… the truth is that fabrics with higher thread counts are meaningless and do not reflect quality in any way.