- Procedures and functions are used to define reusable subprograms. Procedures do not return a value while functions return a value. - Packages are used to group related types, constants, and subprograms. Package declarations define the interface while package bodies define the implementation details. - VHDL describes both the structure and behavior of hardware designs. Structure is defined using entities, architectures, blocks, and components. Behavior is defined using processes, signals, and assignments.

1. Subprograms and Packages