Free-content works are those other than software which are licensed freely in the same sense as free software is licensed freely. That is to say, recipients are given permission, or freedom, to use the content for any purpose, copy it, modify it, and to redistribute modified versions.
Like free software licences, free-content licences can be copyleft, where distributing modified works is only allowed under the original Free license, or non-copyleft. A public domain work does not have a current license (e.g., a work can be in the public domain because its copyright has expired), but many definitions would include public domain works as free content.
The Design Science License (DSL) and GNU Free Documentation License (GFDL) are examples of copyleft licenses for free content. The FreeBSD Documentation License is an example of a non-copyleft license. The GFDL is the license being used for the text of Wikipedia.
Other examples of free-content licenses are some of those published by Creative Commons, when commercial use and derivative works are not restricted. Note, however, that Creative Commons defines a set of licenses, and not all of those licenses are "free-content" as defined here.