UML (Unified Modeling Language) is a standardized modeling language used to visualize, specify, construct, and document artifacts of a software system. It provides a common language for describing systems across different stakeholders. UML includes various building blocks like classes, interfaces, use cases, etc. and relationships between them. It also defines diagram types like class diagrams, sequence diagrams, etc. to group related elements. UML aims to be precise, unambiguous, and complete in modeling systems.