The monk fruit, siraitia grosvenorii, is native to China and parts of Thailand. The primary use for this fruit in the U.S. is for a very, almost no-cal sweetener. The sweetener is made from the juice and pulp of the fruit. In China the fruit pulp is eaten and the thin skin is used to make tea.
Monk fruit is 100-250 times sweeter than granulated sugar so keep this in mind when you start using it in place of regular table sugar. Sugar producer, In The Raw suggests using their monk fruit sugar in beverages, added to smoothies or even to sweeten your yogurt or to top your cereal.
Sugar plays an important roll in baking including adding flavor, helping baked goods to brown and adding volume. In The Raw suggests experimenting with your favorite recipes and substitute not more than 1/2 of table sugar with monk fruit sweetener. This sweetener is heat stable so you can bake or cook with it.
If you don't have monk fruit sweetener you can substitute another low calorie sweetener such as Stevia or Splenda. Substituting sweeteners can be a little tricky and involves some trial and error. For best results use recipes that written to use that specific sweetener.
If you recipe calls for Stevia but you want to use Monk Fruit, you need to know what form of stevia the recipe calls for. For example is it a liquid sweetener where a couple of drops equals a tablespoon of sugar or a powder that is a 1:1 substitute for sugar? Unfortunately recipes are not always forthcoming with the details about the sweetener they are suggesting. Once you know each sweeteners equivalents you can match the amounts suggested by the recipe. By equivalents I mean how much of each sweetener does it take to equal 1 tablespoon of sugar?
Most likely you won't find the fruit in your local grocery store although you may find them if you have A Chinese or Thai market in your area. Otherwise you can purchase monk fruit paste or monk fruit sugar in many well-stocked markets such as Whole Foods. You can also purchase them online at Amazon.com: Monk fruit puree and sugar.