Maybe you have heard the phrase "quality over quantity" before. Well, this goes for searching the web, too.
Finding information online provides a lot of quantity (millions of results), but the quality of what you find can vary drastically.
The argument, information, project, etc. on which you are working is only as strong as the evidence you bring to it.
Your writing or presentation takes on the character of your sources. If you use unreliable sources, your own paper will be unreliable and unbelievable.
Like periodicals, most websites fall into six major categories:
Organizational sites attempting to influence public opinion. Mostly useful for analyzing the organization in question.
Commercial sites often provide information, with the larger motive of selling you something.
Company or organizational attempt to provide current information as a public service. These sites typically include advertisements and may ask you to subscribe for full access.
These sites tend to present factual and statistical information. However, the reliability may be variable and requires critical viewing on your part to verify the information.
Most of the web is devoted to entertainment; these include sites about games, music, movies, television, fiction, and pornography.
Individual home pages are usually for the promotion of individuals, their ideas, or their work, which may be entertaining, informative, or useless.
REMINDER: Essentially anyone can publish pretty much whatever they want to the web. This is why it's so important to critically evaluate online sources of information!