This document defines and explains several key configuration directives for Apache web servers: SSI directives allow adding dynamic content to HTML pages on the server; the document root specifies the directory for web pages; the error log path logs error messages; the listen port specifies the TCP port for HTTP requests; the server root sets the server's directory; and virtual hosts enclose directives for a particular website configured on the server.

1. APACHE
SSI(Server Side Includes):
SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated on the
server while the pages are being served. They let you add dynamically generated content to an
existing HTML page, without having to serve the entire page via a CGI program, or other dynamic
technology.
The decision of when to use SSI, and when to have your page entirely generated by some
program, is usually a matter of how much of the page is static, and how much needs to be
recalculated every time the page is served. SSI is a great way to add small pieces of information,
such as the current time. But if a majority of your page is being generated at the time that it is
served, you need to look for some other solution.
Document root:
The directory (in the real filesystem) from which your Web server will be serving most of its
Web pages.
For example:
If you set the document root to /home/httpd/html, then accesses to
http://your.webserver.com/index.html would return the file /home/httpd/html/index.html. An access
to http://your.webserver.com/foo/gazonk.gif would return /home/httpd/html/foo/gazonk.gif.
Error log :
The path to the log file for error messages. Usually, it is logs/error_log, which is relative
to the ServerRoot. Often the directory logs in ServerRoot is a symbolic link to /var/log/httpd. Then
the log path above would result in error messages being logged to /var/log/httpd/error_log.
Listen on port:
The TCP port on which the Web server should listen for HTTP requests. The standard
port for HTTP is 80, so if you use another port you need to include it in the URL. For example, if
you let your Web server listen on port 8000, then the URL to your server would be
http://your.webserver.com:8000/.
Server Root:
The ServerRoot directive sets the directory in which the server lives. Typically it will
contain the subdirectories conf/ and logs/. Relative paths in other configuration directives (such as
Include or LoadModule, for example) are taken as relative to this directory.
Example:
ServerRoot /home/httpd
Virtuat Host:
<VirtualHost> and </VirtualHost> are used to enclose a group of directives that will
apply only to a particular virtual host. Any directive that is allowed in a virtual host context may be
used. When the server receives a request for a document on a particular virtual host, it uses the
configuration directives enclosed in the <VirtualHost> section.