The Chicago Tribune defines a blog thusly:

“A Weblog is a Web site that maintains a constantly updated list of links to other sites; those links can deal with any subject or focus on a particular one. Webloggers typically offer pithy, sarcastic commentary about the links.”

Or here’s another one:

“A ‘blog’ is short form for ‘web log’, a sort of online personal diary where *anyone* can reveal his/her innermost thoughts, feelings, desires, dreams, ambitions, hopes, fears. A blog lists posts ordered by date, in a personal journal format, and is easier to create than a website – just involves typing into a form and hitting the PUBLISH button.”

Darren Rowse defines a blog as:

‘a website in which items are posted on a regular basis and displayed in reverse chronological order’ – that is, the newer posts appear first, and older ones later.

Pick the blog definition that works best for you.