Internet Copyright Laws - Basics
Copyright Laws offer protection for intellectual property online in the same way that they protect more physical versions. Note that copyright protection applies to many different forms of content by default. It is no longer necessary to explicitly label a work as copyrighted or file for a copyright with the United States Copyright Office (although filing a work with them is usually a good idea for major works where legal trouble is expected).
What this means for Internet copyright issues is that most works online are under some form of protection unless otherwise stated.
Written works are completely protected. You can't copy someone's work without their permission or at least a citation (if it falls under fair use, see below). There are some exceptions. Naturally someone can't claim copyright infringement for a universal fact. Another exception is the issue of titles. A title can't be copyrighted under U.S. law. For example, if someone else wrote an article with the title "What Everyone Should Know about Copyright Laws: Internet Tips & Guides," but wrote a unique article, it would not be infringement. As you can see, written works generally have common sense copyright laws.
Copyright protection applies to unique websites and website templates, although that's naturally a fuzzy area except in cases of direct copying. In the case of a third-party designer, the rights to the website's design are usually signed over to the website's owner.
Images and videos also fall under fairly common sense laws. They belong to the original producer (and the original producer needs to take care of obtaining releases and not violating any privacy laws while filming or photographing). You cannot take a photo or video from someone and host it as your own online. Simply noting that it isn't yours isn't necessarily enough to remove the Internet copyright issue. The media belongs to either the original producer or the website it is hosted on. Some image and video hosts allow users to use their media elsewhere as long as their rules are followed.
For example, Morguefile.com provides completely free images that don't even need an attribution. Flickr.com has a more complicated set of rules, since it allows users to set the license for their image. Some allow anyone to use it as long as the image is linked back to them, while other have stricter regulations. In general, before you take an image or video from a website, check their creative commons or licensing policy.