- Use case diagrams describe relationships between actors and use cases, where actors represent roles users play and use cases represent activities within a system. Common relationships include generalization/specialization between use cases, extension of use cases, and inclusion of use cases within other use cases. - For example, in an auction website, the "Place Bid" use case could be extended by a "Pay with Credit Card" use case or included within a "Checkout" use case. Actor roles like "Customer" and "Administrator" may also be specialized. - Use case diagrams can reference other diagrams to show relationships at different levels of detail.