VHDL code is composed of at least 3 fundamental sections:
1) LIBRARY declarations: Contains a list of all libraries to be used in the design. For example: ieee, std, work, etc.
2) ENTITY: Speci?es the I/O pins of the circuit.
3) ARCHITECTURE: Contains the VHDL code proper, which describes how the circuit should behave (function).
A LIBRARY is a collection of commonly used pieces of code. Placing such pieces inside a library allows them to be reused or shared by other designs.
原文：http://www.cnblogs.com/mengdie/p/4483103.html