Open Source Software (OSS) means that the source code as well as the binary (the actual program you run) is accessible and distributed freely.
Free software may cost money and may be covered under a public license, the most common of which is the GNU Public License (GPL).
Open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology. Before the term open source became widely adopted, developers and producers used a variety of phrases to describe the concept. Opening the source code enabled a self-enhancing diversity of production models, communication paths, and interactive communities. Subsequently, a new, three-word phrase "open source software" was born to describe the environment that the new copyright, licensing, domain, and consumer issues created.