A Use Case is where a specific function of a product is utilized by a persona (a user for example). Bringing all the use cases together creates an assembly of all the possible functions of the product as a whole. A Use Case details the entirety of a function carried out by a persona and the manner in which the persona interfaces with the product.
Use Cases are specific to single events or functions of the product. When multiple use cases are combined, they can form a scenario. From here a scenario can tell a story and can be illustrated as such.