It works the same as wire gauges, the higher the number the smaller the size. When analyzing the yarn count, the ply and yarn size are import. This refers to the direction of the twisting. All yarns are twisted to keep the fibers together. A 2 ply has 2 ends twisted together.
Yarns can have a high or a low twist.
High twists have more twists per inch than a low twist yarn. A high twist yarn generally is smaller than a low twist yarn. Yarn is 1 color. A 2 ply yarn with each ply a different color. Each ply is dyed a color, then twisted to form a 2 ply yarn.