A blog is a collection of relatively simple technologies that evolved to facilitate individual publishing. There are many technologies, strategies and designs that could factor into a website being labeled as a blog but in it's simplest definition a blog is a webpage with content added and listed chronologically. The newest information comes first and the oldest gets pushed down. Information is often grouped by categories and archived by date.
Blogging is facilitated by a simple Content Management System that allows for authors to enter their posts, title them and then publish them. Other features commonly associated with blogs are syndication feeds (RSS, Atom or RSS2), blogrolls, comments, trackbacks & categories, although it is entirely possible for a site to lack one or more of these features and still generally be called a blog. It is equally possible for a site to have 1 or more of these features and not generally be labelled a blog.
It might be best to describe a blog as being a flexible, informal vehicle to publish.