According to Wikipedia, a blog is "a contraction of the term 'web log' and is a type of website, usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order."
There are many different types of blogs, including: personal blogs, corporate/organization blogs, or subject blogs.
Blogs can be monitored individually, using web browser bookmarks, or aggregated via an RSS newsreader service.
A 3-minute YouTube video.