Syslog:

  Syslog, the system logger, is a simple (yet flexible) way for administrators
  to control the logging of the applications under their control.  It
  requires that the programmers of the applications have used the syslog API.
  The basic idea behind syslog is that applications can be classified into
  categories (syslog calls them "facilities").  Syslog then builds upon this
  by letting an application pick its facility, and then log at different
  levels.  These levels are designed to allow applications to indicate which
  messages are more important than others.

  The facilities that syslog provides are (this is just copied from the
  syslog(3) manual page):

       authpriv      - security/authorization messages
       cron          - clock daemon (cron and at)
       daemon        - other system daemons
       kern          - kernel messages
       local0-local7 - reserved for local use
       lpr           - line printer subsystem
       mail          - mail subsystem
       news          - news subsystem
       syslog        - messages generated internally by syslogd
       user(default) - generic user-level messages
       uucp          - UUCP subsystem

  These facilities are fixed.  There is no way to add more.  But local0-local7
  provide most of the flexibility that is required --- many non-core applications
  provide a configuration mechanism that admins can use to change the syslog
  facility to whatever is appropriate for their site.

  The levels that syslog provides are (this is also just copied from the manual
  page):

       emerg   - system is unusable
       alert   - action must be taken immediately
       crit    - critical conditions
       err     - error conditions
       warning - warning conditions
       notice  - normal, but significant, condition
       info    - informational message
       debug   - debug-level message


  These levels are also fixed.  There is no way to add more.

  The syslog daemon reads a configuration file, syslog.conf, which contains
  a series of entries of the form:

    facility.level action

  Each of these lines indicates that messages logged to facility "facility"
  that were logged at the level "level" *or higher* should be subject to
  the action "action".  The two main actions that are provided are to send
  the messages to a file (the action line will just be a file name), or to
  the syslog daemon on a remote host (the action line will be @remotehost).
  The facility and level can also be substituted with a * --- which means "any".

  More advanced syslog daemons can understand the following in the syslog.conf
  file:

    facility.=level --- matches messages at level "level" to facility "facility"
                        (compared to anything at level "level" or above)
    |action         --- send selected messages to stdin of command "action".

  but you shouldn't count on these working on every system.

  Example:

  A common, and simple setup for a site might have messages above and at the
  info level being logged to a central loghost, and being sent to console:

  $ cat /etc/syslog.conf
  *.info  @loghost
  *.info  /dev/console
  $