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. This idea has been reinvented several times in the last 10 years. Each vertical line indicates an existing object. When it is active it the line becomes a rectangle. Normally the steps are described on the left hand side of the diagram.