A blog is traditionally just a website with a stream of recent articles on the main page and links to chronologically-sorted and categorized articles. Blog authors and administrators usually create content and manage the site through a web browser or mobile app.
Not limited to just simple articles, blogs can facilitate posting of custom structured content types, such as status updates, asides, FAQs, how-tos, gallery posts, recipes, reviews, video diaries, etc.
With blogs, site visitors sometimes have the option to interact with the post authors and site visitors via a comment system. However, because of maintenance headaches caused by spammers and trolls, many public websites have chosen to disable comments.
A site powered by a blog content management system can be indistinguishable from one driven by an enterprise content management system. Sometimes you need a service like builtwith.com to help you figure out if a site was built with WordPress.