Slide 68 of 93
VHDL provides the package mechanism so that user-defined types, subprograms, constants, aliases, etc. can be defined once and reused in the description of multiple VHDL components.
VHDL libraries are collections of packages, entities, and architectures. The use of libraries allows the organization of the design task into any logical partition the user chooses (e.g. component libraries, package libraries to house reusable functions and type declarations).