"D" stands for Denier. It's a numbering system for fibers, filaments and yarns, in which the lower numbers are lighter/finer and the higher numbers heavier/coarser. "T" stands for Thread Count – specifically the number of warp and fill threads in a square inch. The lower numbers represent a loosely woven fabric and the higher number a tightly woven fabric. These two numbers together help indicate the strength and feel of a piece of fabric.