Confused about the differences between Free Software, Open Source Software and Proprietry Software?
Here is a brief explanation, but for a more detailed and accurate account you may wish to visit the Free Software Foundation's Philosophy of the GNU Project page.
'Free Software' is software which has been fully released to the public by it's author with the intention that anyone has the freedom to use, share, copy, study or modify the software. There is generally just one significant condition: all modifications must be made freely available under the same free licence.
A GNU or GNU style licence is typically used to prevent 'Free Software' from reverting to closed or proprietry software. It is this provision which strongly encourages further development and has led to rapid, high quality improvement of 'Free Software' by people and organisations from all parts of the world. Popular software is usually actively supported and any faults or problems are typically rectified very quickly, often within days of a release.
It is possible to sell 'Free Software', but because anyone can legally copy and share the software, the price is obviously very low, typically reflecting just the convenience value of it being made available on media such as CD's or DVD's. Nearly all 'Free Software' is accessible on the Internet. It can usually be downloaded without charge, but since it is often quite large it may be very costly and/or slow to download.
'Free Software' by definition is Open Source Software (OSS); but the reverse cannot be said to be true.
All software or computer programs licensed as OSS, have their functional design fully revealed to the public by being made available in their 'source code' form. Unlike with 'Free Software' under GNU or similar licences however, some OSS may contain license restrictions about the way in which the software may be used or modified, and it may be necessary to pay for access to the source code. Perhaps, for example, the author or owner of OSS is releasing details of the software mainly so another person can easily see how to connect and communicate with the software and thus write accessories of their own. Such an author however may still wish to prohibit copying or modification of the original software, and may put these restrictions into the OSS license.
Proprietry Software on the other hand, which most people presently use, is typified by heavily restrictive licence provisions which prohibit copying or modification under any circumstances, and which even prohibit multiple use in the same family or business, unless extra licences are purchased. These provisions usually lead to very high prices; so high that alternative annual subscriptions with fringe benefits are now being used as a ploy to encourage the unwary purchaser to commit to a purchase. Serious faults in the software are usually rectified by 'patches', which may take a long time to be made available. Minor faults and poor design usually have to wait for the next version which must then be paid for again.
A very serious problem with proprietry software is the erosion of standards, which if fostered and observed, would otherwise permit users to share documents even if they had computers or software of different brands or age. In the absence of government defined standards, this has resulted in profit motivated vendors very successfully locking government, education, consumers and businesses into continuously updating with the same brand of software, through fear of incompatibility if a change is made.