As you create reusable procedures, you can place them in a library file for convenient access. Although you can put any routine into a library, each library usually contains related routines. For example, you might place string-manipulation functions in one library, matrix calculations in another, and port communications in another.
A library consists of combined object modules, each created from a single source file. The object module is the smallest independent unit in a library. If you link with one symbol in a module, you get the entire module, but not the entire library.
A library can consist of two files—an include file containing necessary declarations and constants and a .LIB file containing procedures already assembled into object code.