Information from web sites can be very easy to find. Due to the nature of the web, it is very important to evaluate anything that you find before including it in an assignment or essay.
Anyone can author or publish material on a web site. No quality control process is required.
This doesn't mean that you shouldn't use information from web sites just that you need to apply an evaluation criteria to understand:
Authority - What type of site is it? Who authored the content? What was the intended audience?
Objectivity - Is the purpose and intention of the source clear, including any bias or particular viewpoint?
Accuracy - Fact or opinion? What evidence is presented? Does the site reference other sources?
Currency - When was the last time the site was updated? Does the information presented have a "published" date?