The letters PO in .po files means Portable Object, to distinguish it from .mo files, where MO stands for Machine Object. This paradigm, as well as the PO file format, is inspired by the NLS standard developed by Uniforum, and first implemented by Sun in their Solaris system.
PO files are meant to be read and edited by humans, and associate each
original, translatable string of a given package with its translation
in a particular target language. A single PO file is dedicated to
a single target language. If a package supports many languages,
there is one such PO file per language supported, and each package
has its own set of PO files. These PO files are best created by
xgettext program, and later updated or refreshed through
msgmerge program. Program
xgettext extracts all
marked messages from a set of C files and initializes a PO file with
empty translations. Program
msgmerge takes care of adjusting
PO files between releases of the corresponding sources, commenting
obsolete entries, initializing new ones, and updating all source
line references. Files ending with .pot are kind of base
translation files found in distributions, in PO file format.
MO files are meant to be read by programs, and are binary in nature.
A few systems already offer tools for creating and handling MO files
as part of the Native Language Support coming with the system, but the
format of these MO files is often different from system to system,
and non-portable. The tools already provided with these systems don't
support all the features of GNU
gettext. Therefore GNU
gettext uses its own format for MO files. Files ending with
.gmo are really MO files, when it is known that these files use
the GNU format.