Free content (or free information) is any kind of functional work, artwork, or other creative content upon which no legal restriction has been placed that significantly interferes with people's freedom to use, understand, redistribute, improve, and share the content. Importantly, when free content is modified, expanded, or incorporated within another work, the resulting work must be legally distributable, either just as freely as or under more restrictive terms than the original. Free content, whether available at no cost or only for a fee, must be able to be changed if it may be used at all.
Practically speaking, free content encompasses all works in the public domain and also those copyrighted works whose licences honour and uphold the freedoms mentioned above. Because the law by default grants copyright holders monopolistic control over their creations, copyrighted content must be explicitly declared free, usually by the referencing or inclusion of licensing statements from within the work.
A work in the public domain cannot be licensed because, by definition, its copyright has expired or been relinquished. However, such a work is still considered free content, because it may be used for any purpose whatsoever.
Free-content licenses may be copyleft -- in which case modifications of the work must themselves be distributed only under the terms of the original free license -- or else they are non-copyleft, which means that the licensed work may be modified and then distributed under a different license, even one that is less free.
Most free content licenses contain provisions specifying that derivative works must attribute or give credit to the authors of the original, a requirement which promotes intellectual honesty and discourages plagiarism without imposing so great a burden as to weaken the claim of such licenses to being truly free.
The Design Science License (DSL) and GNU Free Documentation License (GFDL) are copyleft licenses for free content. The FreeBSD Documentation License is an example of a non-copyleft license. The GNU General Public License (GPL) can also be used as a free content license.
Other examples of free content licenses are some of those published by Creative Commons when commercial use and derivative works are not restricted, although they do not require a "source" copy of the license be provided. Note that not all Creative Commons licenses are free content as defined here.
Libre and gratis
Besides free as in freedom, there is also another important meaning of the word free: free of charge. The two meanings of the term free are often illustrated with the phrases "free as in beer," which alludes to monetary price or cost but has little to do with freedom, and "free as in speech," which alludes to the widely-recognized freedom of speech (see, for example, the First Amendment to the United States Constitution), but which has little to do with monetary price or cost. The usage of "free" in "free content" carries the latter meaning -- free as in speech -- because the emphasis is on everyone's freedom to engage with the content, understand it, modify it, and share it with others.
Many languages other than English use two different words for these distinct concepts. In English, it is sometimes useful to use two less common but more precise words, both adopted from French: libre (meaning free as in speech) and gratis (meaning free as in beer). In these terms, free-content works are always libre but not necessarily gratis.
Free content and open content
Free content licenses differ from open content licenses in that they require a "source" copy of the content to be provided. For example, a free content publisher should make the source document (f.ex. InDesign or word-processor file) available along with a PDF, which in this case would be considered the "object" copy of the creative work. Some free content licenses have stronger requirements. For example, the GNU Free Documentation License not only requires that a "source" copy of the content is provided, but that the source copy should be in an "transparent" format, i.e., in an open format whose specification is freely available to everybody.