A (web)log of chronologically ordered posts that usually are available to the public (though many blogs - especially personal journals and class blogs - are password-protected and private)
Traditionally text-based, though also may include (or be solely based on) images, video, other multimedia.
Usually available via subscription, via email (notification of new posts comes to your inbox) or RSS feed. Subscription increases convenience to the reader: you don't have to visit sites of interest in search of something new. New content comes to you.
What is a blog, anyway?
In a nutshell, a blog is:
Some types of uses/focus areas: