Next to proprietary software which can be used, distributed and modified exclusively under an authorization granted by a licence, there are programs, amongst which Linux, which are called free software, or more precisely software “open source”, which can be used, copied, redistributed and modified without any restrictions.
The term “free” does not refer to the free software, that can be distributed in return for payment, but to the fact that the code id open and accessible.
The substantial difference between proprietary software and open programs is that in the latter the source code is at the user disposal whereas it does not concern the price that should be paid for their purchase.
Theoretically a proprietary software might be distributed free of charge and vice versa open source software might have to be paid for.
Among proprietary software programs there are shareware programs, which can be copied and distributed against a royalty payment, but not modified, as their code is unavailable; and freeware programs that do not require any royalty for their distribution.
Even for these latter ones, the source code is not accessible to the user.
The “open” software can be differentiated between copylefted, meaning that can be modified but still protected by the initial copyright, therefore no restrictions can be added at the time of redistribution, and no-copylefted, meaning that they can be redistributed by anyone, whether modified or not.
Even when you use an open source software or freeware program, it is recommended to carefully read the conditions written in the licence, because not paying for a program does not inevitably mean unlimited exploitation of the program.
The licences, in fact, always establish specific restrictions to be respected by the user.