Open-Source Software vs. Free Software: Although the terms are often used interchangeably, OSS is slightly different from free software. Both deal with the ability to download and modify software without restriction or charge.
However, free software a concept developed in the 1980s by an MIT computer science researcher, Richard Stallman is defined by four conditions, as outlined by the nonprofit Free Software Foundation. These “four freedoms” emphasize the ability of users to use and enjoy software as they see fit.
In contrast, the OSS criteria, which the Open Source Initiative developed a decade later, place more emphasis on the modification of software, and the consequences of altering source code, licensing, and distribution.
Obviously, the two overlap; some would say the differences between OSS and free software are more philosophical than practical. However, neither should be confused with freeware. Freeware usually refers to proprietary software that users can download at no cost, but whose source code cannot be changed.