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Apache - A web server

A web server's job is basically to accept requests from clients and send responses to those requests. A web server gets a URL, translates it to a filename (for static requests), and sends that file back over the internet from the local disk, or it translates it to a program name (for dynamic requests), executes it, and then sends the output of that program back over the internet to the requesting party. If for any reason, the web server was not able to process and complete the request, it instead returns an error message. The word, web server, can refer to the machine (computer/hardware) itself, or the software that receives requests and sends out responses.
Popular web server - Why?

• It is free to download and install.
• It is open source: the source code is visible to anyone and everyone, which basically enables anyone (who can rise up to the challenge) to adjust the code, optimize it, and fix errors and security holes. People can add new features and write new modules.
• It suits all needs: Apache can be used for small websites of one or two pages, or huge websites of hundreds and thousands of pages, serving millions of regular visitors each month. It can serve both static and dynamic content.

Functionality that you don't need or want can easily be removed.
The Apache HTTP server is a software (or program) that runs in the background under an appropriate operating system, which supports multi-tasking, and provides services to other applications that connect to it, such as client web browsers. It was first developed to work with Linux/Unix operating systems, but was later adapted to work under other systems, including Windows and Mac. The Apache binary running under UNIX is called HTTPd (short for HTTP daemon), and under win32 is called Apache.exe.

Apache's original core is fairly basic and contains a limited number of features. Its power rather comes from added functionality introduced through many modules that are written by programmers and can be installed to extend the server’s capabilities. To add a new module, all you need to do is install it and restart the Apache server. Functionality that you don’t need or want can easily be removed which is actually considered a good practice as it keeps the server small and light, starts faster, consumes less system resources and memory, and makes the server less prone to security holes. The Apache server also supports third party modules, some of which have been added to Apache 2 as permanent features. The Apache server very easily integrates with other open source applications, such as PHP and MySQL, making it even more powerful than it already is.

Web server
A web server in its simplest form is a computer with special software, and an internet connection that allows it to connect to other devices.

The Apache server offers a number of services that clients might make use of. These services are offered using various protocols through different ports, and include: hypertext transfer protocol (HTTP), typically through port 80, simple mail transfer protocol (SMTP), typically through port 25, domain name service (DNS) for mapping domain names to their corresponding IP addresses, genearlly through port 53, and file transfer protocol (FTP) for uploading and downloading files, usually through port 21.

Directories constitute Apache
As we know, Apache can be installed on a variety of operating systems. Regardless of the platform used, a hosted website will typically have four main directories: htdocs, conf, logs, cgi-bin.

htdocs is the default Apache web server document directory, meaning it is the public directory whose contents are usually available for clients connecting through the web. It contains all static pages and dynamic content to be served once an HTTP request for them is received. Since files and sub-directories under htdocs are available to the public, correct handling of file permissions is of great importance so as not to compromise the server’s safety and security.

conf is the directory where all server configuration files are located. Configuration files are basically plain text files where directives are added to control the web server’s behavior and functionality.

logs is the directory where server logs are kept, and includes Apache access logs and error logs. The Apache HTTP Server provides a variety of different mechanisms for logging everything that happens on it, from the initial request, through the URL mapping process, to the final resolution of the connection, including any errors that may have occurred in the process.

cgi-bin is the directory where CGI scripts are kept. The CGI (Common Gateway Interface) defines a way for a web server to interact with external content-generating programs, which are often referred to as CGI programs or CGI scripts. These are programs or shell scripts that are written to be executed by Apache on behalf of its clients.

Note:   It is important to note that the above discussed file and directory names (as well as locations) can differ from one server to another depending on the Apache flavor installed and the operating system it runs under. The roles though remain the same.

Enjoy Programming!!!