A version of this page appears in Share This Book.
Describes a work that can be shared and adapted without limitations, though with conditions
A libre work can be shared and adapted by anyone in the world.
When the creator places their work under a libre licence, they give permission for everyone now and in the future to share and adapt their work. This permission, once given, cannot be withdrawn.
This permission is unlimited. You can share and adapt their work no matter who you are or how you are sharing it. You can sell it, print it out, put it on a file-sharing network, and so on.
This permission is conditional. When adapting their work, you have to obey certain conditions. The most popular is attribution: if you share or adapt a work, you have to give credit to the original creator. Another is a copyleft restriction. If you adapt a copyleft work, you must place your adaptation under the same copyleft licence.
These conditions do not apply to the original creator. If your book is under an attribution condition, you don’t have to credit yourself. Everyone else must unless they have your permission.
Libre is very closely related to free software, open source and F/OSS. ‘Open source’ and ‘free software’ are used by two different ideological movements in the libre community. I don’t want to pick a side.
In summary: If it’s a libre work, you can share and adapt it without paying royalties or asking anyone for permission.
Not all licences fit the strict libre definition given above. There are other licences which still only reserve some rights, but reserve too many to qualify as libre.
The most obvious examples are works that can be shared and adapted for noncommercial purposes only and works that can be shared but not adapted (that is, they must be shared verbatim). I describe these works as noncommercial common content and verbatim common content, respectively, or as proprietary common content to describe both.
Common content itself consists of proprietary common content and libre. Its definition is: ‘Works that can be, at the very least, shared verbatim for noncommercial purposes’.