Entity declaration: It defines the names. Input output signals and modes of a hardware module. Also it provides the external interface of an entity. It is a black box view.
entity entity _ name is
Architecture: It describes the internal description of it tells what is thereinside design. Each entity has at least one architecture and an entity can have many architecture. Architecture can be described using structural, dataflow, behavioral or mixed style. Architecture can be used to described a design at different levels of abstraction like gate level (RTL) or behavior level.
architecture architecture _name of entity_ name
Architecture_ declaration_ name;
end architecture_ name;