I like Wikipedia’s description of a blog. In it, they say:
A blog is a website in which items are posted on a regular basis and displayed in reverse chronological order. The term blog is a shortened form of weblog or web log. Authoring a blog, maintaining a blog or adding an article to an existing blog is called “blogging”. Individual articles on a blog are called “blog posts,” “posts” or “entries”. A person who posts these entries is called a “blogger”.
Pretty simple, but they go on to talk all about how blogging and websites differ, dynamics of blogs, types of blogs, you name it.