Definition : a blog is a web site, where you write stuff on an ongoing basis. New stuff shows up at the top, so your visitors can read what's new. Then they comment on it or link to it or email you. Or not.
Useful as a communications and writing tool
What makes blogs special? Can’t all of this be done with websites, newsgroups, and other existing web technologies? Blogs blend features of different web technologies into a new form. What makes blogs unique is that they are inexpensive, easy to create, and dynamic. The big thing is that blogs make creating dynamic websites available to everyone.