Copyright law was created to protect original art whose value could only be measured in its uniqueness, things that had no practical value in the world. It sadly exists today as a weapon of corporations that want you to believe that their rights are supreme, and they wield them with an indiscriminate and often overbearing hand.
Luckily for the public, the Congress saw fit to include exceptions in the law that allowed the public to copy protected works in a balance of fairness, but you wouldn't know it if you've ever watched a football or baseball game on television.
"The proceeding program is owned by the National Football League; descriptions of events, blah blah, and blah may not be broadcast, redistributed, blah blah, in any form without the consent of the National Football League."
Statements like these deprive people of the ignorance defense, but also serve to misinform the public of their rights regarding the television broadcast. Any moderately large media holder makes claims like these that have no basis in law, and you can find more examples in the Associated Press reports we are bombarded with every day.
This copyright statement makes it clear who owns the story and who to contact if you wish to license it. 'All rights reserved' is a statement of convenience for foreign intellectual property treaties, but it is the last part that is pretty much a blatent lie.
17 U.S.C. 107 is the section of the Copyright Act that gives the public exemptions in the form of four tests, which determine the impact on the work being copied and the fairness of its usage.
This first test and the fourth have ascended above the others in recent years as copyright has increasingly been usurped by corporations to protect valuable assets instead of being an incentive to create art. The United States Supreme Court has gone on record saying that any use which passes these two tests must be presumed to be fair use without the necessity of applying the remaining two.
If the portion you are copying is being used in a school text to teach, or a news report to inform, in some way having social commentary (parody) then you will pass this test. If you are using the material in a commercial, the noose will tighten and you will have a much harder time gaining exception.
This test is often applied when a person or entity collects together a number of "facts", and then claims copyright on them as a whole as an expressive work of creativity.
The courts have ruled that facts cannot be copyrighted (your name, your address, or your favorite color individually) but that some collections can be (name, address, phone number = phonebook.)
Live sports scores fall under this category as "factual events" and have recently been considered inapplicable to copyright.
This is probably the most often cited test as it relates to the quotation of written pieces of literature. News and magazine articles and sometimes book reviews that include samples will fall under this test.
There are no hard numbers that say "if you only copy 10% of the original, than you are free and clear," and the other tests can have an effect on how much is too much as well. If you are quoting from a news article in something that you yourself are selling, then the amount you can safely use is probably going to be very small when considered by the courts. Commercial use always makes fair use much more strict.
It is best practice when unsure to use no more than a paragraph or two at most, while copying the entire work will most certainly come under very strict scrutiny (not to be confused with "strict scrutiny".)
It is useful to note that Government documents in the United States are typically not copyrighted at all, and may be used freely.
This fourth and final test is very important when it comes to things such as fan fiction. Does a market exist for the work you are copying from? Might one exist in the future? Are you competing with the author/owner in that market for money?
If any of these questions is answered in the affirmative, then you will probably be in some sort of trouble. If the answer is no, you are likely to prevail, but will probably still have to see the trial through all the way to the end as the finding of facts won't happen for a summary judgment motion.
A good rule of thumb in fair use is to consider what you are doing and how an outside person may perceive your actions. If you are copying a paragraph from an AP report while writing about the same subject, perhaps not as a journalist would but still within the realm of contemporary social discussion, then you are pretty safe. Two or three paragraphs may be stretching your rights a bit thin, but if you are not competing against the AP in writing actual news and are not borrowing their work in an attempt to make a profit, you can get away with using a little bit more. If you find yourself copying half or more, you are probably in violation of the law.
Images work a little differently, because you must copy the entire work for it to be of any use. The courts have been split over whether or not a thumbnail constitutes a small use, so you are rolling the dice no matter what you do. Journalists have far more freedom in this area than the rest of us do, due to academic exceptions and value. And remember, as far as courts are concerned, there is no such thing as the citizen journalist when it comes to the law.
There are some myths about how you can copyright something in the United States, such as placing a manuscript into an envelope and then mailing it back to yourself. I've personally seen this done and I can tell you with complete confidence that it holds no legal value.
Be as creative as you wish, the law only recognizes the copyright of a work when it has been properly submitted to the Copyright Office and later the Library of Congress. While it is true in the United States that your work attains copyright from the moment it has been created, you absolutely cannot sue over infringement until it has been registered, and if you end up doing this, you will not be eligible for statutory damages.
In all likelihood, while you will not be sued in your lifetime for copyright infringement, think about how you would feel if you found your work copied unfairly and without permission, and remember that just because you found it on the Internet, that doesn't mean it is free and yours for the taking.
For those who wish for their writings to be available for others to use with fewer restrictions than copyright law may generally allow, but do not wish to hire a lawyer to lay them out in a useful way, you may wish to begin using a Creative Commons license, as I have done with this article.
I have chosen a license which requires that anyone who copies this text attribute it to me by name, and chose just a couple of restrictions such as no commercial use and no creation of derivative works. In exchange, the license allows unlimited copying in part or in whole without permission of any kind, forever.
The license does not remove your rights to use my text as granted under the fair use clause, so it's really just a clear way to tell people that you can use this stuff more freely than otherwise, but you have to play nicely.
Copyright may be a weapon to companies like the RIAA, but for us it can also be a shield. Know your rights under the law, and use them to brighten our world.