Typically, tallow starts with the extraction of suet from a carcass. Suet is hard fat found in the neighborhood of the kidneys and around some other organs. While suet can be used as-is, rendering it removes the impurities and also extends the shelf life. Once suet is rendered, it becomes tallow. As long as it is stored in an airtight container in a cool environment, it can keep for an extended period of time, unlike suet, which will become rancid.