Open source software (OSS) began as a marketing campaign for free software. OSS can be defined as computer software for which the human-readable source code is made available under a copyright license (or arrangement such as the public domain) that meets the Open Source Definition. This permits users to use, change, and improve the software, and to redistribute it in modified or unmodified form. It is often developed in a public, collaborative manner. Open source software is the most prominent example of open source development and often compared to user generated content.

The free software movement was launched in 1983. In 1998, a group of individuals advocated that the term free software be replaced by open source software (OSS) as an expression which is less ambiguous and more comfortable for the corporate world. Software developers may want to publish their software with an open source license, so that anybody may also develop the same software or understand how it works. Open source software generally allows anyone to make a new version of the software, port it to new operating systems and processor architectures, share it with others or market it. The aim of open source is to let the product be more understandable, modifiable, duplicatable,reliable or simply accessible, while it is still marketable.

The debate over open source vs. closed source (alternatively called proprietary software) is sometimes heated.

Some closed-source advocates see open source software as damaging to the market of commercial software. This is one of the many reasons, that the term free software was replaced with open source - because many company executives could not believe in a product that did not participate economically in a free-market or mixed-market economy. In addition, if something goes wrong there is the difficult question of who is liable.

The counter to this argument is the use of open source software to fuel the market for a separate product or service. For example:

Providing support and installation services; similar to IT Security groups, Linux Distributions, and Systems companies.
Using the software as a stepping stone to sell a higher-end product or service; e.g., OpenOffice.org vs. StarOffice.
Cost avoidance / cost sharing: many developers need a product, so it makes sense to share development costs (X Window System and the Apache web server)
Another major argument is software defects and security: This is an argument that applies to all open products not just open source software.

Since Open Source software is open, all of the defects and security flaws are easily found. Closed-source advocates argue that this makes it easier for a malicious person to discover security flaws. Further, that there is no incentive for an open-source product to be patched. Open-source advocates argue that this makes it easier also for a patch to be found and that the closed-source argument is security through obscurity, which this form of security will eventually fail, often without anyone knowing of the failure. Further, that just because there is not an immediate financial incentive to patch a product, does not mean there is not any incentive to patch a product. Further, if the patch is that significant to the user, having the source code, the user can technically patch the problem themselves. These arguments are hard to prove. However, most studies show that open-source software does have a higher flaw discovery, quicker flaw discovery, and quicker turn around on patches.

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