In general, copyright law is defined by the international Berne Convention for the Protection of Literary and Artistic Works, which covers written materials, photography, illustrations, sketches, etc., but doesn't cover "news of the day or to miscellaneous facts having the character of mere items of press information".

A few important points from the Berne Convention: Copyright lasts for the life of the author and fifty years after his or her death. There's also Fair Use, which allows for (in typically vague legal wording) "quotations from a work which has already been lawfully made available to the public, providing that their making is compatible with fair practice".

Here's the most important paragraph in this long international agreement though: "In order that the author of a literary or artistic work protected by this Convention shall, in the absence of proof to the contrary, be regarded as such, and consequently be entitled to institute infringement proceedings in the countries of the Union, it shall be sufficient for his name to appear on the work in the usual manner." [ref]

This means that you do not need to have an explicit copyright notice for you to have a legally enforceable copyright, as long as your name appears on the work as an author. Anonymous material is covered too, but you can read about that yourself if you're interested.

The US Copyright Office clarifies how the Berne Convention applies to U.S. Copyright law: "The use of a copyright notice is no longer required under U.S. law, although it is often beneficial."

Continuing to quote from the Copyright Office: "Use of the notice may be important because it informs the public that the work is protected by copyright, identifies the copyright owner, and shows the year of first publication. Furthermore, in the event that a work is infringed, if the work carries a proper notice, the court will not give any weight to a defendant's interposition of an innocent infringement defense, that is, that he or she did not realize that the work was protected. An innocent infringement defense may result in a reduction in damages that the copyright owner would otherwise receive." [ref]

Finally, if you do decide to include a copyright notice - and I would certainly recommend it - then it must have three elements:

1. The symbol © (the letter C in a circle) or the word "Copyright" or the abbreviation "Copr.". Notice that the sequence of a parenthesized capital C (that is, (C)) is not a valid copyright symbol.
2. The year of first publication
3. The name of the owner of the copyright in the work, or an abbreviation by which the name can be recognized.

An example of a good copyright is © 2005 Dave Taylor. (tip: you can get the copyright symbol in HTML by using the character entity ©)

Finally, Web sites that have extensive original content might find it valuable to register their copyrighted material with the U.S. Copyright Office, though it's more of a legal formality and is not a condition of copyright protection under the law. If you do pursue this avenue, start by reading about Registering Literary Works, wherein you'll find out that it costs a mere $30 per element for copyright registration.

Tip: If you want to register your entire Web site, print it all out in book form, then register the entire collection of articles as a book for a single $30 payment, rather than $30 per page or article. If it's a Weblog, you might find that registering it under the aegis of Serials and Periodicals is your best strategy.

Finally, this is a lot of information to absorb, and even as a professional writer I get confused about things too, particularly about the vaguely worded "fair use" clauses in copyright law. Nonetheless, a bit of additional protection for your work seems like a smart strategy, so I'd recommend that you include a full, legal, copyright notice on all your published content.