'Fairy shrimp' is the general name given to species within the order Anostraca, but is usually only used when reffering to the freshwater species. The freshwater species cysts are usually sold in low quantities at a realativly high price on ebay as betta or killifish fry food. They require distilled, RO or rain water to hatch properly.
Artemia is a genus of fairy shrimp (as said, the order Anostraca) that live in salt water, usually called brine shrimp. Sea-monkeys are an Artemia species too.
These are a much beter option for feeding baby fish, as you can buy hundreds of thousands of cysts very cheaply, and they will hatch in pretty much any water with around 30 grams of salt (aquarium, sea or even cooking) per litre.