A piping bag is kitchen equipment used for squeezing dough, batters, creams, and puréed components like potatoes onto platters or baking sheets, as well as for decorating cakes, pies, cookies, and other pastries.
In general, a piping bag is a bag that is shaped like a cone. The substance to be piped is placed in the bag, which is then twisted to drive the ingredient toward the pointed end of the cone. The open pointy end can be fitted with any number of tips to create a variety of decorative effects.
Piping bags come in a variety of sizes, ranging from very large for huge assignments to very little for highly detailed work on cakes and biscuits. For cream puffs or eclairs, pastry cream is piped through a bag, and choux pastry dough is piped onto a baking sheet. Piping bags are also used to fill pastries, such as cream-filled doughnuts or jelly-filled doughnuts.