Use Cases vs. Sequence Diagrams — Level of Detail

Use case models provide a high-level overview of an interaction; sequence diagrams add more detail by showing the specific objects involved and the sequence of messages between them

Explanation

Use case models and sequence diagrams present interactions at different levels of detail and are used together. A use case gives a high-level overview of an interaction (what happens). The details of the interactions involved in a use case can then be documented in a sequence diagram (how it happens — which objects are involved, in what order, with what parameters and return values).