自由軟件（free software）
"Free software" means software that respects users' freedom and community. Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software. Thus, "free software" is a matter of liberty, not price [1].
"Free software" gives users access to source code. Users have the rights to use the software, redistribute it, modify it, and distribute their modifications. Users can receive payment for these distributions. However, since any user can redistribute as many copies of the software as they would like, the price is normally very close to the cost of media. Other than the copylefted* conditions below there are virtually no restrictions on the use of the software or how it can be modified. This includes primarily public domain software and copylefted software[2].
自由軟件關注的重點在於用戶對軟件的絕對支配權，可以自由地運行、復制、散布、學習、修改並改進該軟件，而不在於軟件是免費與否。如果一個軟件為自由軟件，那么它需要提供以下四種自由度[1]：
Freedom 0: The freedom to run the program as you wish, for any purpose. 無論何種目的，用戶可以自由地運行該軟件。
Freedom 1: The freedom to study how the program works, and change it so it does your computing as you wish. Access to the source code is a precondition for this. 用戶可以自由地學習並修改該軟件用於實現用戶想得到的計算。前提是用戶得到軟件的源代碼。
Freedom 2: The freedom to redistribute copies so you can help others. 用戶可以自由地散布該軟件的復制用於幫助別人。
Freedom 3: The freedom to distribute copies of your modified versions to others. By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precodition for this. 用戶可以自由地散布該軟件修改后的復制，因此整個社區都可以得益於修改后的軟件。前提是用於得到軟件的源代碼。
可見，訪問軟件源代碼是其成為自由軟件的必要條件。用戶可以自由地修改軟件並散布給其他人而不需要任何許可。通常自由軟件都比非自由軟件更加可靠。
開放源代碼軟件（open source software）
"Open source software" is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licenses that we consider too restrictive, and there are free software licenses they have not accepted. However, the differences in extension of the category are small: we know of only a few cases of source code that is open source but not free. In principle it could happen that some free programs are rejected as open source, but we don't know if that has ever happened[3].
開放源代碼軟件與自由軟件的含義有些類似，因此也經常被混淆。但是從上面可以看出，開放源代碼軟件只是自由軟件的一部分，並不能完全替代自由軟件。開發者更傾向於使用“自由軟件”的概念。
公共軟件（public domain software）
"Public domain software" is source code that has no restrictions on use or distribution. It can be used in any way, copied, modified, distributed, and sold with any added licensing terms desired[2].
"Public domain software" is software that is not copyrighted. If the source code is in the public domain, that is a special case of non-copylefted free software, which means that some copies or modified versions may not be free at all[3].
公共軟件沒有版權，因此修改后的軟件被添加了一些許可后就不再是自由的了。因此可以說公共軟件不是copylefted的。
私有軟件（proprietary software）
“Proprietary software” is the traditional commercial model. Software executables are sold or leased with no user right to distribute the executable to others. Commercial licensing terms very, but limited copying rights may apply for backups or for other fair uses that do not deprive the software vendor from rightful income under its license. Source code is not provided under this model[2].
私有軟件不提供源代碼，用戶也沒有散布、復制、修改的權利。
共享軟件（shareware）
Shareware executables have no restrictions on distribution, but users are required to pay for the software under the honor system if they decide to use it. Source code is not provided[2].
Shareware is software which comes with permission for people to redistribute copies, but says that anyone who continues to use a copy is required to pay a license fee[3].
共享軟件不限制軟件的散布，但是不提供源代碼，想要使用該軟件需要支付費用。因此共享軟件並不是自由軟件。
版權所無軟件（copylefted software）
"Copylefted software" is free software that has been copyrighted so that the users rights modify and redistribute the source code are always transferred to subsequent users[2].
"Copylefted software" is free software whose distribution terms ensure that all copies of all versions carry more or less the same distribution terms[3].
版權所無軟件是一種自由軟件，用戶修改后的軟件在散布時要遵守原始軟件的條款和許可。GNU發布的General Public License (GPL)是一個特定的版權所無許可，用以保證自由軟件對所有用戶是自由的。
Reference
[1].https://www.gnu.org/philosophy/free-sw.en.html
[2].Shackleford W P, Proctor F M. Use of open source distribution for a machine tool controller[C]//Sensors and Controls for Intelligent Manufacturing. International Society for Optics and Photonics, 2001, 4191: 19-30.
[3].https://www.gnu.org/philosophy/categories.en.html
*Copyleft 著佐權，是一個由自由軟件運動所發展的概念，是一種利用現有著作權體制來保護所有用戶和二次開發者的自由的授權方式。在自由軟件授權方式中增加著佐權條款后，該自由軟件除了允許使用者自由使用、散步、修改之外，著佐權許可證更要求使用者修改后的衍生作品必須要以同等的授權方式釋出以回饋社會。——百度百科