What is a Use case and an Actor?

A Usecase represents a particular scenario which corresponds to functional requirement(s) of a system to be designed and developed.An Actor is a user/external program or a system(anyone or anything), which interacts with a system.An Actor may input/receive or both(input and receive) information from the system.

In the diagram shown below, a scenario of buying bus/train/tram ticket from a vending machine is captured through a use case diagram.Here actor is a 'Customer' as shown by a stickman and in ovals all usecases have been documented.