The following extracted from Wikipedia:
"Shirataki are very low carbohydrate, low calorie, thin, translucent, gelatinous traditional Japanese noodles made from the konjac plant. The word "shirataki" means "white waterfall", describing the appearance of these noodles. Largely composed of water and glucomannan, a water-soluble dietary fiber, they have little flavor of their own.
Shirataki noodles can be found both in dry and soft "wet" forms in Asian markets and some supermarkets. When wet, they are purchased pre-packaged in liquid. They normally have a shelf life of up to one year. Some brands may require rinsing or par-boiling as the water they are packaged in has an odor that may be unpleasant to those not accustomed to it.
There are two types of shirataki noodles sold in the United States. Traditional shirataki noodles have zero net carbohydrates, zero calories, no gluten, and are useful for those on low-carbohydrate diets. Tofu-based shirataki-style noodles are becoming increasingly popular in U.S. supermarkets and health food stores. They have a much shorter shelf life and require refrigeration even before opening. Tofu-based noodles contain a minimal amount of carbohydrates."