Thread count (TC) refers to the number of fibres woven together in one square inch of fabric.  Thread counts can range anywhere from 80-1500TC.  Thread Count is very important when choosing good quality sheets although thread count alone is not the only factor to consider when purchasing a set of sheets.  One must consider the type of cotton used and the "ply" of the sheets.  The term "ply" refers to the number of single fibres twisted together before it is woven into a fabric and by twisting two fibres together mills can double the thread count of a fabric.  By twisting three fibres together they can triple it and so on.  Most 1000TC sets of sheets are actually 330TC 3-ply or 250TC 4-ply and generally the higher the ply the heavier and thicker a set of sheets are.  If you prefer a heavier set of sheets, then a high thread count with a high ply (3-4) is for you, they are also quite warm to sleep under!  (All sheets above 600TC are at least 2-ply).  If you're looking for the ultimate in softness and the feel of a lighter sheet then a high thread count with low (single)  ply (300TC, 600TC) is for you!  The highest thread count that can be woven into a 'single ply' is 600TC and if you can find a set of these then you've found yourself a great set of sheets especially if they are Egyptian Cotton.  Most people usually find anything above 300 Thread Count 'single ply' has the luxury and softness they are after!