Note that this is an essay about copyright myths. It assumes you know at least what copyright is — basically the legal exclusive right of the author of a creative work to control the copying of that work. If you didn’t know that, check out my own brief introduction to copyright for more information. Feel free to link to this document, no need to ask me. Really, NO need to ask.
1) “If it doesn’t have a copyright notice, it’s not copyrighted.” This was true in the past, but today almost all major nations follow the Berne copyright convention. For example, in the USA, almost everything created privately and originally after April 1, 1989 is copyrighted and protected whether it has a notice or not. The default you should assume for other people’s works is that they are copyrighted and may not be copied unless you know otherwise. There are some old works that lost protection without notice, but frankly you should not risk it unless you know for sure.
It is true that a notice strengthens the protection, by warning people, and by allowing one to get more and different damages, but it is not necessary. If it looks copyrighted, you should assume it is. This applies to pictures, too. You may not scan pictures from magazines and post them to the net, and if you come upon something unknown, you shouldn’t post that either.
The correct form for a notice is:
“Copyright [dates] by [author/owner]”
You can use C in a circle © instead of “Copyright” but “(C)” has never been given legal force. The phrase “All Rights Reserved” used to be required in some nations but is now not legally needed most places. In some countries it may help preserve some of the “moral rights.”
2) “If I don’t charge for it, it’s not a violation.” False. Whether you charge can affect the damages awarded in court, but that’s main difference under the law. It’s still a violation if you give it away — and there can still be serious damages if you hurt the commercial value of the property. There is a USA exception for personal copying of music, which is not a violation, though courts seem to have said that doesn’t include widescale anonymous personal copying as Napster. If the work has no commercial value, the violation is mostly technical and is unlikely to result in legal action. Fair use determinations (see below) do sometimes depend on the involvement of money.
3) “If it’s posted to Usenet it’s in the public domain.” False. Nothing modern and creative is in the public domain anymore unless the owner explicitly puts it in the public domain(*). Explicitly, as in you have a note from the author/owner saying, “I grant this to the public domain.” Those exact words or words very much like them.