in short (from WIKI) Many blogs provide commentary or news on a particular subject; others function as more personal online diaries. A typical blog combines text, images, and links to other blogs.
My personal difficulty is the lack of clarification as to which is commentary and which is news. In addition I find a difficulty finding original sources for validation of content on many blogs that simply circle endlessly in reference to themselves or to other blogs. I'm pretty sure there are many credible blogs however, I find it more and more difficult to determine the wheat from the chafe so to speak. ie. is the new to be build mosque to be erected ON ground zero or a few blocks away on land currently owned and occupied by Muslims. Some blogs have it one way, some have it the other. Opinion pieces are fine when so identified, not so fine when presented as factual.