Open Source SoftwareOpen source software is one of the larger categories of free software and it is free in more than one way. By definition, the source code (human readable program) is made available to anyone and may be modified by anyone who agrees to make their coding public also. That characteristic is often described as "free as in freedom" due to the freedom to modify the code.
Most, but no all, open source software is also "free as in beer" meaning it is given away with no payment required like being given free beer. For most of us this the "free" that we really care about because we can get some very useful or fun software for free. However, open source software is often of high quality because many very good programmers collaborate and share their skills and hard work.
FreewareSome people call open source software freeware, but generally, freeware is defined as software that is available for free, but the source code is usually not shared. Most freeware programs are provided by individual programmers or companies that wish to share their software with the public. Freeware my be provided as a way to market other commercial programs or the author may just want to share the software with others.
Freeware often has more restrictions on use than open source. Many freeware authors don't allow their software to be used by businesses for free and may even sell a professional version for business use.
SharewareProgrammers and companies sometimes make their programs available for a limited trial period. Shareware is usually fully functional and may or may not have a "nag screen" that encourages you to register the program. The trial period is usually two weeks or thirty days. Shareware games usually offer more levels or features when you pay to register them. Shareware is a recognized way to market software without the overhead of a publishing company.
Demos and Trial SoftwareTrial software and demo software are usually more limited in functionality than shareware. Often a trial business application will allow you to enter data but not print, or save a file. Game demos are usually only one or two levels just to give you a taste of the game.
Abandonware and Liberated SoftwareThese terms usually refer to software or games that are no longer sold and supported by the publisher, but are available for free either unofficially or officially. The former term refers to software that has literally been abandoned and the later to software that the publisher has deliberately released to the public for free.
Public Domain SoftwareWith all the other types of free software there is a copyright or license restrictions of some type. When software is released as public domain it means the creator of the work gives up ownership and anyone is free to do whatever they wish with the software. Typically, software is given to the public domain when the author wants people to be free to use it but has no interest in making money from it or supporting it in any way.
As you can see there are many ways that software can be made available for free and I didn't even talk about the darker side of free software such as adware, spyware, and illegal or pirated software. I'll leave those topics for another time and another article.