Open Source usually refers to software that is released with source code under a license that ensures that derivative works will also be available as source code, protects certain rights of the original authors, and prohibits restrictions on how the software can be used or who can use it.

The source code of an open source computer program has to be freely available  to the users.  The software developers who support the open source concept believe that by allowing anyone who's interested to modify the source code, the application will be more useful and error-free over the long term.

To be considered as open source software by the software development industry, certain criteria must be met:

• Anyone must be allowed to modify the source code.
• The license must not require the exclusion of other software or interfere with the operation of other software.

The most important difference between software created by the open source communities and commercial software sold by vendors is that open source software is published under licenses that ensure that the source code is available to everyone to inspect, change, download, and explore as they wish.
One or more developers--meaning people who have the skills to create software--get an idea about creating software to solve a problem.

Usually the source code is published fewer than one of several popular open source licenses that ensure that the source code and any derivative works remain open source. A huge amount of amazing software has been created through this loose process.

Examples of leading open source applications are:

Linux: one of the most prominent examples of free software and open source development; typically all underlying source code can be freely modified, used, and redistributed by anyone. The name "Linux" comes from the Linux kernel started in 1991 by Linus Torvalds. The system's utilities and libraries come from the GNU operating system, started in 1983 by Richard Stallman. Predominantly known for its use in servers, Linux is supported by corporations such as Dell, Hewlett-Packard, IBM, Novell, Oracle Corporation, Red Hat, and Sun Microsystems. It is used as an operating system for a wide variety of computer hardware, including desktop computers, supercomputers, video game systems, such as the PlayStation 2 and PlayStation 3, several arcade games, and embedded devices such as mobile phones and routers.

MySQL: is a multithreaded, multi-user SQL database management system (DBMS) which has, according to MySQL AB, more than 10 million installations. The basic program runs as a server providing multi-user access to a number of databases. MySQL is popular for web applications and acts as the database component of the LAMP, MAMP, and WAMP platforms (Linux/Mac/Windows-Apache-MySQL-PHP/Perl/Python), and for open-source bug tracking tools like Bugzilla. Its popularity for use with web applications is closely tied to the popularity of PHP and Ruby on Rails, which are often combined with MySQL. PHP and MySQL are essential components for running popular content management systems such as Joomla. Word Press and Drupal. Wikipedia runs on Media Wiki software, which is written in PHP and uses a MySQL database.

Mozilla: Firefox is a web browser project descended from the Mozilla application suite, managed by the Mozilla Corporation. Firefox had 16.80% of the recorded market share in Web browsers as of December 2007, making it the second-most-popular browser in current use worldwide. Firefox uses the open-source Gecko layout engine, which implements current Web standards plus a few features which are intended to anticipate likely additions to the standards. Firefox runs on various versions of Microsoft Windows, Mac OS X and Linux. Its current stable release is version 2.0.0.11, released on 30 November 2007. Firefox's source code is under the terms of the Mozilla tri-license as free and open source software.

PHP: is a computer scripting language originally designed for producing dynamic web pages. The name PHP is recursive initialism for PHP: Hypertext Preprocessor. PHP is used mainly in server-side scripting, but can be used from a command line interface or in standalone graphical applications. Textual User Interfaces can also be created using ncurses. The main implementation is produced by The PHP Group and released under the PHP License. This implementation serves to define a de facto standard for PHP, as there is no formal specification. The most recent version of PHP is 5.2.5, released on 8 November 2007. It is considered to be free software by the Free Software Foundation.

Open Office: is an office suite application available for a number of different computer operating systems. It supports the Open Document standard for data interchange as its default file formats, as well as Microsoft Office '97-2003 formats, among many others. OpenOffice.org is based on Star Office, an office suite developed by Star Division and acquired by Sun Microsystems in August 1999. The source code of the suite was released in July 2000 with the aim of reducing the dominant market share of Microsoft Office by providing a free, open and high-quality alternative. OpenOffice.org is free software, available under the GNU Lesser General Public License (LGPL).