Does anyone know the difference between _pic.a and .a libraries?

Also, looking at /usr/src/share/mk/bsd.README,

MKPICLIB        If "no", don't build *_pic.a libraries, and build the
                shared object libraries from the .a libraries.
                A symlink is installed in ${DESTDIR}/usr/lib for the
                _pic.a library pointing to the .a library.
                Default: yes

what will be the difference between shared objects built with _pic.a 
libraries and object built with .a libraries?

Thanks