What is a 'Blog'?

Blog is short for 'weblog'.  It's basically a website that's frequently updated.  The newest blog entry, or post, is usually displayed first, and older posts are usually archived in one or more ways (by age, category, author, etc.).  Even after a post is archived, it's always accessible through its 'permalink' (a link to the post's permanent location).  Blogs are usually informal publications.  Many blogs allow readers to make 'comments', which are also displayed with the post.  A blog can be used for many purposes: a personal diary, a professional publication, a knowledge base, etc.