Libraries contain code which is often required by more than one program. This has the advantage that each program doesn't need to duplicate code (and risk introducing bugs), it just has to call functions from the libraries installed on the system. The most obvious example of a set of libraries is Glibc which is installed during the LFS book. This contains all of the C library functions which programs use.
There are two types of libraries: static and shared. Shared libraries
(usually libXXX.so) are loaded into memory from the
shared copy at runtime (hence the name). Static libraries (
libXXX.a) are actually linked into the program executable file
itself, thus making the program file larger. Quite often, you will find
both static and shared copies of the same library on your system.
Generally, you only need to install libraries when you are
installing software that needs the functionality they supply. In
the BLFS book, each package is presented with a list of (known)
dependencies. Thus, you can figure out which libraries you need to have
before installing that program. If you are installing something without
using BLFS instructions, usually the README or
INSTALL file will contain
details of the program's requirements.
There are certain libraries which nearly everyone will need at some point. In this chapter these and some others are listed and it is explained why you may want to install them.