Open Source is the process of producing a software product with source code freely and openly available to one and all who wish to see it or modify it. The group of people who do this is called the Open Source community, although it should be noted that there is rarely a single identifiable community that shares exactly the same position on every single aspect of software development and licensing. The Open Source Initiative is the primary advocacy organisation for Open Source software.
Predating the name Open Source is the Free Software movement. The two communities are very much aligned, but Open Source avoids talking about principles of freedom in favour of appealing to business interests, typically emphasizing the supposed benefits of an open development model such as its ability to produce higher quality products with superior security and reliability records.