Blogs can range from extremely informative and high quality to mere low-budget advertisement. The company that created SweetSearch called Finding Dulcinea gives you some basic questions to ask yourself about any blog to determine if it is credible or not.
Wikipedia is a multilingual, web-based, free-content encyclopedia project based on a model of open editable content. It is written collaboratively by volunteers. The page linked is from the Wikimedia Foundation on whether Wikipedia is credible for use academically.
A URL or Uniform Resource Locator is the location of a file on the web. It is like the address of a web page. Every URL has a domain extension. Here are two URL extension guides to assist you in determining the type of site you are visiting.