Filet crochet is characterized by using very fine cotton thread, size 10, 20 or 30 (the larger the number the finer the thread).   Here in Panama it is a very common craft, used for making decorative borders for baby blankets or doing baby booties.

The hooks used are also very small.  They go from size 0 to 7 (or 9).  In Panama, the largest crochet hook you can find is a size 7, I've had to import the larger sized hooks to make my own crochet work (from B to K, P, N and Q)