GNU License and Copyright Collection

What is copyright left
www.gnu.org/copyleft/copyleft.ru.html
www.gnu.org/copyleft/copyleft.html

Free and non-free software categories
www.gnu.org/philosophy/categories.ru.html
www.gnu.org/philosophy/categories.html
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BSD license issue
www.gnu.org/philosophy/bsd.ru.html
www.gnu.org/philosophy/bsd.html

Free software safer
www.gnu.org/software/reliability.ru.html
www.gnu.org/software/reliability.html

Why "open source" does not convey the concept of free software
www.gnu.org/philosophy/open-source-misses-the-point.ru.html
www.gnu.org/philosophy/open-source-misses-the-point.html

Why copyright left
www.gnu.org/philosophy/why-copyleft.ru.html
www.gnu.org/philosophy/why-copyleft.html

FAQ under the GNU license
www.gnu.org/licenses/gpl-faq.html

In short, everyone has long known that there is a difference between open source software and free software (let alone free software). Free software implies source code that is accessible to all, but not vice versa, although quite often these terms are used to describe the same thing. Free software is just free and does not always apply to free software.

There is also the notion of a program in the public domain. In essence, these are programs without exclusive copyright. What is most interesting is that the majority of free programs are not deprived of copyright, but have a certain owner. But the use of free licenses gives access to both the program itself and the source code.
Copyright left is a kind of restriction that does not allow to turn a program into a non-free one in the process of modifying or distributing it. Therefore, all licenses are divided into 2 groups - with and without copyleft. Often copyleft prohibits the use of free code for commercial purposes - when someone takes your available code and uses it for their own selfish interests.