In 1991, the United States Supreme Court ruled in the case of Feist Publications, Inc., v. Rural Telephone Service Co., Inc. (499 US 340) in which Feist copied information from Rural's telephone listings to include in its own, after Rural refused to license the information. Rural sued for copyright infringement. The court ruled that information contained in Rural's phone directory was not copyrightable, and that therefore no infringement existed.
It is a long-standing principle of American copyright law that "information" is not copyrightable, but "collections" of information can be. Rural claimed a collection copyright in its directory. The court clarified that the intent of copyright law was not, as claimed by Rural and some lower courts, to reward the efforts of persons collecting information, but rather to encourage creative expression. Its opinion stated that collection copyrights apply to only the creative aspects of collection: the creative choice of what data to include or exclude, the order and style in which the information is presented, etc., but not on the information itself. The court ruled that Rural's directory was nothing more than an alphabetic list of all subsribers to its service, and that no creative expression was involved. The fact that Rural spent considerable time and money collecting the data was irrelevant to copyright law, and Rural's copyright claim was dismissed.
This has major implications for such projects as an internet encyclopedia. Information (that is facts, discoveries, etc.), from any source, is fair game. But you do have to make sure that it does not contain any of the "expressive" content added by the source author. That includes not only the author's own comments, but also his choice of which facts to cover, his choice of which links to make among the bits of information, his order of presentation (unless it is something obvious like an alphabetical list), any evaluations he may have made about the quality of various pieces of information, or anything else that might be considered "original creative work" of the author rather than mere facts. For example, a recipe is a process, and not copyrightable, but the words used to describe it are. You can therefore rewrite a recipe in your own words and publish it without violating copyrights. But if you rewrote every recipe from a particular cookbook, you might still be found to have violated the author's copyright in his choice of recipes and their "coordination" and "presentation", even if you used none of his words. As another example, the text of US Laws is in the public domain, but West Publishing Corporation claims a copyright on the page numbers in its printed edition of those laws! So you can reference a law, and even include large exerpts with impunity, but if you reproduce it in such as way as to deliberately preserve West's page numbers, you might be in trouble (this case is still pending). You can make a reference to page numbers in West, as in the first paragraph of this article (the notation 499 US 340 is legal shorthand for "Volume 499 of West's US Reports, page 340").