About Free, Libre and Open
There have been and still are intense flamewars about the use of the terms free, libre and open in the context of software, knowledge and licensing. As the Free Software movement was the first in defining the main concepts (of copyleft, using copyright to protect the freedoms and thus stimulate sharing of knowledge), let us start there.
Free Software was first fully defined on January 1989 in the GNU's bulletins, and in 1992 "Libre Software" was proposed as a synonym based on the same definition. In 1997 Free Software was specified in the Debian Free Software Guidelines (DFSG), and in 1998 the DFSG were used as the definition of the term "Open Source" which was proposed as a marketing term for Free Software, later resulting in the combined terms "FOSS" and "FLOSS".
The term "Open Source" got associated with a "pragmatical, more business oriented" view, even though Free Software leaders, such as Richard Stallman, have made it clear both in their communications as in their actions, that Free Software doesn't stop anyone doing business, but the contrary: it gives you precisely that freedom, without depending on others for permission. In general, the term "Open Source" is proposed as being apolitical: where the Free Software movement can be seen as in part a political movement, grounded in very strong ethical values, Open Source is defined as a development methodology ("many eyeballs make any bug shallow"), which takes away the struggle for freedom that can be clearly identified in the Free Software movement. While most Free Software is also Open Source and viceversa, the development methodogy is in many of these software projects indeed an open development method, although in fact the development methodology is not formally one of the defining criteria of neither of them (FSF Definition, Open Source Definition).
Some people use the term "libre", which refers to liberty. As in English "free" has been misunderstood as it has both the meaning of freedom and "for free", libre might be less confusing, is used in Spanish and comes and from the same latin root as for example "livre" in french and portuguese, and "libero" in italian. Free Knowledge has thus the same meaning as Libre Knowledge, Free Software as Libre Software, free licenses as libre licenses and so on.
Some people use the term FLOSS (Free, Libre, Open Source Software), as being the perfect politically correct term that would cover all three movements. Though indeed it covers all three of them, it doesn't take a position, nor does it bring clarity. Abreviations need to be explained as much or more than any of the individual terms, so it can be questioned how much uninformed readers will understand from it.
The term open is generally used for referring to open access to something, be it an open door that provides access to the space behind it, or as "open publication" on the internet, without any restriction on access, or in an "open process", where participation is allowed and encouraged for all participants. Free Software and Free Knowledge in general are in most of the cases developed in "open development communities", i.e. mostly online communities where interested people participate in the design, implementation, testing and distribution of explicit forms of knowledge. In the case of software, access to the source code is a precondition for such open development practice. However, more than only open access to the source code, all interested participants (users) need to have the four freedoms that define Free Software to enable an effective open process. Therefore the term "Open Source" brings in some confusion, as one would derive from the name that only open access to the source code would suffice the software to be called "Open Source Software". This, however, is not true, as Open Source Software is defined by 10 criteria, which ultimately require the software to adhere to the four freedoms. Note that most software that is considered Free Software is also Open Source Software and vice versa. This is assured by the accepted licenses by the Free Software Foundation and the Open Source Initiative respectively. Note also that Free Software (and Open Source for that matter) can also be developed in a closed process, for example, if one developer (or development team) writes code secretely and publishes this under a free license.
We generalise from free software to free knowledge, and indicate the importance of the semantics in building community and shaping the future - towards a broad vision for a free knowledge society. Free Knowledge is explicit knowledge made available with the freedom to use, study, adapt, copy and distribute modified versions. From the Free Knowledge perspective, our focus is on freedom (not "for free"; people who understand these concepts nowadays know the difference), for several reasons. First, consider that free knowledge (under free licenses) in all its forms is (generally) produced by communities in an open process (open participation) and by open publication of inputs and outputs (open access). This is what is also described as commons-based peer production, or peer production of free knowledge. While the freedoms that define free knowledge lead to the production in such open processes and open publication, the other way around is not necessarily the case. In short, freedom is the defining criterion for free, libre and open knowledge, and serves in our view better to convey understanding (see: The Free Knowledge Institute's Founding Principles). Second, the various free, libre and open movements have been greatly inspired by the Free Software movement, which was the first global movement to raise the issues of user freedoms related to software and knowledge. Thirdly, the struggle for freedom at the highest level is what has marked the progress of mankind throughout history, where development means the expansion of the freedoms of man both as an objective and as a means (Amartya Sen, Development as Freedom, 1999).
This said, to avoid splitting the positive energy of millions of people over the term each chooses to use, we consider it a useful habit to introduce the terms in the start of any relevant document and refer to the other two terms at least once (and if so desired make references to relevant websites that explain the concepts in more detail). Even though the political connotations of the terms differ greatly, these movements are also very output oriented, and we can therefore be pragmatical to introduce for example free software as "Free Software (a.k.a. Open Source or Libre Software)" so readers familiar with one of the terms understand we're talking about just that. Whenever possible and relevant, it is recommended to explain in more detail the background of each of the terms, movements and history behind it. That is generally a very good way of raising more awareness.