|PostgreSQL 7.3.2 Programmer's Guide|
libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library routines that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries. libpq is also the underlying engine for several other PostgreSQL application interfaces. So some aspects of libpq's behavior will be important to you if you use one of those packages.