Free content 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, 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. 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 vs Open Content
Free content licenses generally differ from open content licenses in that they require a "source" copy of the content to be provided. For example, a free content publisher could provide a Microsoft Word document as a source for a creative work contained in a PDF document. Some free content licenses have stronger requirements. For example, the GNU Free Documentation License (which is the one that AstrologyNotes uses) not only requires that a "source" copy of the content is provided, but that the source copy should be in a "transparent" format, in other words in an open format whose specification is freely available to everybody.
Free Content Licenses
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 GNU Free Documentation License (GFDL) is a copyleft license for free content. 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.