sendfax [options] phone-number file
mgetty [options] tty
pbm2g3 [options] [pbm-file]
g32pbm [options] [pbm-file]
g3cat [options] [g3-file(s)]
faxspool is used to queue jobs for later transmission by fax. faxspool accepts different input formats, converts them into the proper format required by sendfax and places the jobs into the outgoing spool directory.
faxq displays the current FAX work queue.
faxrm removes one or more jobs from the current fax work queue. You have to specify the job id (returned by faxq) on the command line.
faxrunq processes all the faxes in the fax work queue, and sends them one-by-one. If transmission isn't possible, the job will be retried later. While meant to be run from cron, faxrunq can as well be started from the shell (but you must have access to the fax queue and the modems, which means you must be 'root' or 'uucp')
faxrunqd is a replacement for faxrunq to be used in environments that send a high number of faxes per day, or need more flexibility.
sendfax is the backend used by faxrunq for actually sending the fax. It is not meant to be ever called directly, but can be used in own applications.
faxq-helper is a helper program that is used by faxspool, faxq and faxrm to access the fax spool directory (which is not world-writeable since mgetty version 1.1.29).
mgetty is the part of the program suite that handles incoming calls. Data calls are handled as getty does it (login, shell), while fax calls are handled internally. The received faxes are stored in /var/spool/fax/incoming. See the mgetty(8) manpage for details. mgetty can not be called directly, it must be called by /etc/init.
pbm2g3 is used to convert portable bitmaps to G3 fax data (for sendfax). Do not use the pbmtog3 program from the pbmplus or netpbm toolkit, it will create broken G3 data.
g32pbm is used to convert received faxes (which are stored in G3 format) to portable bitmap format. PBM can be converted to other formats with the named pbmplus toolkit.
g3cat is used to concat multiple G3 files, as cat does with text files.