Open Source Software (“OSS”) is computer software for which the source code is made available, and for which it is usually (some exceptions, rules and restrictions may apply) permitted to study, modify and improve the software. Additionally, users are also normally permitted (again, exceptions, rules and restrictions may apply) to distribute the software in modified or unmodified form.Open Source is not a trademark, so while most people used the term in broadly the same way, sometimes you may find that software described as “Open Source” is actually being distributed on a different licensing basis, etc. To reduce the possibility of confusion, the Open Source Initiative developed a set of 10 criteria (OSD) for Open Source licenses, and those licenses which have been verified as satisfying those critera may be described as OSI Certified.Closely related to Open Source is Free Software, as defined by the Free Software Foundation. Free Software is software that is licensed so as to provide the following four freedoms (numbered from zero), and for which access to the source code is considered a precondition (in particular a precondition for freedoms 1 and 3):0. To run the program, for any purpose1. To study how the program works, and to adapt it for your needs2. To redistribute copies3. To improve the program and to release your improvements to the publicIn practical terms, most Open Source Software may also be considered Free Software and vice-versa, hence they are often placed together in a combined category named either Free/Open-Source Software (“FOSS”) or Free/Libre/Open-Source Software (“FLOSS”).An important point to note about both Open Source and Free Software is that with the exception of public domain software (which is sometimes classified as a subtype of Open Source or Free Software), the software is copyrighted, there are terms and conditions which apply, and that using or distributing the software in violation of the terms and conditions is almost certainly copyright infringement.Please note: The author of this article is NOT a lawyer. This article is NOT intended as, NOR should be construed as legal or professional advice.