The variables that are based on reference types store references to the actual data. The keywords that are used to declare reference types are: 1. Class – Refers to the primary building block for the programs, which is used to encapsulate variables and methods into a single unit. 2. Interface – Contains only the signatures of methods, properties, events, or indexers. 3. Delegate – Refers to a reference type that is used to encapsulate a named or anonymous method.