Open source software, Free software and Freeware are confusing terms to the purchaser because they appear on first sight to be similar but the costs, licensing and support are very different and all are available for a variety of platforms including Linux, Unix and Microsoft.
Open source software (Wikipedia) refers to computer software available with its source code and under an open source license to study, change, and improve its design. Open Source is not necessarily free, suppliers may charge to bundle software into a easy to use distribution or charge for installation and support.
Free software, (Wikipedia) as defined by the Free Software Foundation, is software which can be used, copied, studied, modified and redistributed without restriction. Freedom from such restrictions is central to the concept of "free software", such that the opposite of free software is proprietary software, and not software which is sold for profit, such as commercial software. Free software may sometimes be known as libre software. The usual way for software to be distributed as free software is for the software to be accompanied by a free software license, and the source code of the software to be made available.
Freeware (Wikipedia) contrasts with "free software", due to the different meanings of the word "free". Freeware is described as "Gratis", as in "free beer" and refers to free price, versus "Free Software" referred to as libre software, as in "free speech", which refers to the license freedom. Freeware should be examined carefully to ensure it does not have any restrictive license agreement terms.