Most countries in the world have copyright laws which are based on the international Berne convention. This treaty states that any work is copyrighted at the moment of creation. A work has to be original and needs some form of artistic expression, but these requirements are easy to fulfill. Unless a work is really short (such as a single sentence), or is purely mechanical (such as a list of names), it is copyrighted. In the USA, before adopting the Berne Convention, authors had to register their work with the Copyright Office before they could get copyright protection for their work. This is now no longer true, but it may still be beneficial for US authors, as registering allows them to sue for punitive damages, instead of only for actual damages.
The copyright on a work is valid until a certain period after the author's death. The exact length of this period depends on the country, but can be up to seventy-five years. When the author is unknown, this period starts when the work is first published. The length of this period has changed several times in history, and this has affected the status of many old works. So, when using an old work on your site, don't just follow the current rule, also check what was the law at the time the work was created. And note that if you use a photo of a really old work, this photo is copyrighted independently of the work on the photo.
It is not required to add the famous "©" symbol or a text such as "Copyright by <author>" to copyright the work. These serve only as indicators to others that the work is indeed copyrighted, so that no one can later claim he could not have known this. An appropriate notice on a Webpage would be something like:
Copyright © year by author. All rights reserved.
The year should be the year of creation, possibly with the addition of the year it was last modified. The author can be either the maker of the work, or the company or institution who owns the rights. In many plain-text documents, the © symbol cannot be shown, and the string "(c)" is used instead. This string is not an officially-recognized indicator, although it is by now so common that it probably would be accepted as such if there would ever be a dispute about a work's copyright status. To be on the safe side, you could add the full word "Copyright" to the "(c)" string.
More information on patents, copyright, trademarks and other Internet-related law is available on Iusmentis.com.