A 'Blog' is a shortened form of the words Web log.
A blog is a web site like any other, but it is intended to offer an opinion on something or stories about what is happening with the author.
Blogs started out mostly as online personal diaries, but now they have also been taken up by business as an informal method of communicating with customers and clients as well.
The difference between a standard web site and a blog is the way that stories are 'posted' on to the site. A Blog uses a web based application to publish the story being written.
Another feature of many blogs is the ability of visitors to add comments to the stories.