Static Libraries

Static libraries are simply a collection of ordinary object files; conventionally, static libraries end with the ``.a'' suffix. This collection is created using the ar (archiver) program. Static libraries aren't used as often as they once were, because of the advantages of shared libraries (described below). Still, they're sometimes created, they existed first historically, and they're simpler to explain.

http://www.linux.org/docs/ldp/howto/Program-Library-HOWTO/static-libraries.html