A pastry bag, also known as a piping bag, is a kitchen tool used for decorating and filling pastries, cakes, and other desserts. It is typically made of flexible plastic or cloth and has a pointed end where various tips can be attached. These tips allow for different designs and textures when applying frosting, cream, or dough. To use a pastry bag, you fill it with your desired mixture, then squeeze from the top to control the flow. This tool is essential for creating intricate designs, such as rosettes or borders, and is commonly used in professional baking and home kitchens alike.