A weblog is a hierarchy of text, images, media objects and data, arranged chronologically, that can be viewed in an HTML browser.
You will find many details about the components that make up a weblog, but to me the keys are chronology, usually reverse meaning most recent at the top of the page, voice, generally meaning written by a person, and brevity. The last is not something many consider but I think that good weblog posts are no more than three paragraphs long and I think there is a difference between an essay or a story and a weblog post. A story always has a title, a weblog post may or may not not have a title, and in fact most posts do not have a title.
What became confusing is when people and companies used weblog platforms as content management platforms to create and maintain web sites of an form and fashion and thus if a site were published by Wordpress it automatically was called a blog.