A usecase is a set of similar scenarios all with the same type of user and all ending when the user finds out that the process is complete. The user must begin a usecase and must know when it is complete. A usecase is written out in a natural language like English. UML provides a notation for naming and classifying both actors and usecases. The usecase is indicated by an oval.
Communication is modeled as a simple link between an actor and an usecase.
The generalisation link is also used between Usecases to indicate that one usecase is a special case of another one, or that one usecase makes use of another one. These allow you to work out how your classes can help the user solve problems. Sequence diagrams make the timing of various operations clear. Collaboration diagrams are better for modelling a complex interaction or algorithm. There is no reason why you shouldn't draw both.