Ubuntu's Apache2 default configuration is different from the
upstream default configuration, and split into several files optimized for
interaction with Ubuntu tools. The configuration system is
fully documented in
/usr/share/doc/apache2/README.Debian.gz. Refer to this for the full
documentation. Documentation for the web server itself can be
found by accessing the manual if the apache2-doc
package was installed on this server.

The configuration layout for an Apache2 web server installation on Ubuntu systems is as follows:

/etc/apache2/
|-- apache2.conf
|       `--  ports.conf
|-- mods-enabled
|       |-- *.load
|       `-- *.conf
|-- conf-enabled
|       `-- *.conf
|-- sites-enabled
|       `-- *.conf
          

• apache2.conf is the main configuration
  file. It puts the pieces together by including all remaining configuration
  files when starting up the web server.
• ports.conf is always included from the
  main configuration file. It is used to determine the listening ports for
  incoming connections, and this file can be customized anytime.
• Configuration files in the mods-enabled/,
  conf-enabled/ and sites-enabled/ directories contain
  particular configuration snippets which manage modules, global configuration
  fragments, or virtual host configurations, respectively.
• They are activated by symlinking available
  configuration files from their respective
  *-available/ counterparts. These should be managed
  by using our helpers
  
  a2enmod,
  a2dismod,
  
  
  a2ensite,
  a2dissite,
  
  and
  
  a2enconf,
  a2disconf
  . See their respective man pages for detailed information.
• The binary is called apache2 and is managed using systemd, so to
  start/stop the service use systemctl start apache2 and
  systemctl stop apache2, and use systemctl status apache2
  and journalctl -u apache2 to check status. system
  and apache2ctl can also be used for service management if
  desired.
  Calling /usr/bin/apache2 directly will not work with the
  default configuration.