You may notice the numbers 14x14, 12x14, 3x3 etc on the tarp descriptions.  This is known as the "mesh count".  It is the number of "threads" per inch of the tarp.  So, for example, a 12x14 mesh count is 12 threads per inch going one direction and 14 threads per inch going the other direction.  The higher the number, the more reinforcement the tarp has.