/etc is the directory that holds the essential system and software configuration. Use this tag for questions on the organization of /etc or the syntax of configuration files within /etc. Don't use this tag for questions on the configuration of particular software.
The configuration of the system and most software is kept in various files under /etc or in subdirectories. These files are supposed to be static; runtime configuration is kept under /var/run or /var/lib. An exception to this is the file /etc/mtab. Moreover, the configuration under /etc is often system specific and cannot be shared.
/etc once used to contain some system binaries, but they were moved long ago to /sbin. /etc/rmt of the GNU tar package is a rare remnant. In some sense the same holds for the init scripts under /etc/init.d, although several packages install or expect scripts under specific subdirectories of /etc.
Software packages installed under /usr/local or /opt sometimes use /usr/local/etc resp. /opt/etc to store their configuration.
See also
