A use case is a technique to capture the functional requirements of a system. How is it different from a requirements document? Use Case talk more from the domain and the user perspective rather than using any technical jargon. Assuming that a system interacts with the users called as actors, each use case represents a situation, the interactions between the system and actors involved and the purpose. Use cases are done by business analysts and validated by the customer. A use case is the first step to capturing the requirements and gets translated into other areas of the software development lifecycle. Here is a useful pdf document on use case template from bredemeyer.com.