Beef stock is a savory liquid made by simmering beef bones, along with some meat trimmings and vegetables, in water for an extended time to extract collagen, gelatin, and flavor.
Stock is often confused with broth: stock is built mainly from bones and simmered longer, giving a thicker, more gelatinous liquid, while broth is made mostly from meat and simmered more briefly, giving a lighter, more seasoned liquid. The two are used interchangeably in home cooking but classically play different roles.
Beef stock is the base for sauces, gravies, and braises across many cuisines, from classic French sauces to the broths behind pho and ramen. Its flavor is deep, savory, and umami-rich, varying with the bones used, the length of simmering, and any aromatics added.