Pre-defined VHDL components are instantiated and connected together
Structural descriptions may connect simple gates or complex, abstract components
VHDL provides mechanisms for supporting hierarchical description
VHDL provides mechanisms for describing highly repetitive structures easily
VHDL also supports descriptions based on a component's underlying internal structure. Structural VHDL allows for sub-components to be instantiated and interconnected. It may be noted that a structural description is similar to a netlist. Of course, the description of the sub-components can themselves be structural and/or behavioral in nature.
RASSP E&F Module 11, Structural VHDL, concentrates on VHDL constructs that support structural descriptions.