OPEN SOURCE SOFTWARE
The term "Open Source Software" refers to a licensing model where the software must provide the source code along with the executable program. There are various types of open source licenses, the most common of which is the GPL (GNU Public License).
Everyone is free to use and modify software licensed under the GPL. When GPL software is modified, the source code changes must be made publicly available under the GPL. This promotes community involvement and the improvement of GPL software.
This development process is very different from the proprietary model, where the developer tries to keep his source code secret. The secrecy of the proprietary model means that any bugs or security holes can only be fixed by the developer - and if that developer is not available to fix the problem for any reason, the user must suffer. Worse, because the source code for the program is secret, that means that the format that the data is stored in is also probably secret, preventing the user from migrating to a different software package without great difficulty.
The Open Source development process, however, is more like the traditional scientific model where new work builds on existing work, allowing for much more rapid development. Because anyone interested in a particular software project can access and improve the code, that also means that bugs and security flaws are fixed more quickly. And with the source code available, the data format is known, which means that the user can easily migrate their data to a new program, if desired.