According to wikipedia.org

A blog (a contraction of the term “weblog“) 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. “Blog” can also be used as a verb, meaning to maintain or add content to a blog.

This is essentially what a blog is. Though more and more often these days you will often see blogs that have multiple authors. Other than that, Wikipedia has provided us with an excellent summary of what a blog is.