In the UML, a stereotype is a way of adding detail to any part (element) of a model. It is a way of expressing variation or a usage distinction that tells you more about the original element. For example, the line drawn between an actor and a use case indicates that there is an association between them. We could add the stereotype «communication» to such a line to emphasise the communication that takes place between the two. In practice, this stereotype is left out because it is the only type of association between an actor and a use case.
In general, stereotyping is a recognised way of extending the UML. You can define your own term and place it between the angle brackets (or guillemets: «»). However, there must be some agreement in the team about the existence and documentation of such new terms.
The UML includes some stereotypes that you cannot redefine. Two of them are used to describe dependencies between use cases and these are discussed in Subsections 6.8 and 6.9.