UML (Unified Modeling Language) is a standardized modeling language used in software engineering to visualize the design of a system. There are two main types of UML diagrams: structural diagrams that depict the static elements of a system, and behavioral diagrams that show the dynamic interactions between structural elements over time. Behavioral diagrams include sequence diagrams, activity diagrams, and state machine diagrams. Sequence diagrams specifically depict the sequential order of interactions between objects in a system through message passing and lifelines.