user_defined_types let you create custom data shapes beyond primitives. Common kinds include structs, classes, enums, unions, and type_aliases; names vary by language. They bundle data as fields and behavior as methods. constructors set up new objects or records. enums restrict values to a fixed set. type_aliases provide readable names for complex types. Visibility is controlled by access_modifiers like public or private. Remember that user-defined types improve code organization, express intent, and make models closer to real domains.