Technorati monitors the blogophere, and this graphic summarizes their findings in 2008:
While Blogs began as personal diaries, they are now widely used in organizations. An organization blog may be used to communicate with people inside the organization or outside stakeholders. For example, Intel uses blogs to allow specialized communication with employees. Engineers who utilize Intel chips are able to communicate directly with the product managers and engineers who design them. Sun Microsystems CEO Jonathan Schwartz aims his blog at the entire customer base. He describe new products, Sun strategy, Sun's success stories, etc. As you see here, there are many bloggers at Sun at all levels. (Sun and other companies typically encourage employees to blog, but have a few guidelines, for example, not discussing unannounced products. Here are many more examples of public blogs in Fortune 500 companies.
David Pogue recently wrote a New York Times column with examples of the way a blog can increase contact between a large company and its customers.
Blogs are also being used in teaching. For example, the IS Department at Brigham Young University has a site with blogs for students, faculty and alumni. (Check the left-hand column of the home page to see what users can do). We have our class blog, but I would like to see you invent other ways for us to use blogs.
Soon after the Web protocols were invented and graphic browsers became available, people began keeping online diaries on Web sites. At first they were hand coded HTML, but it soon became clear that these diaries had some common features, and people began to write programs to make blog maintenance easier. These evolved into today's fully-networked blog applications in which both the software to create and maintain a blog and the blog data are resident on the Internet.
Today these tools have reached a high level of abstraction. One can get started by simply specifying a few parameter values -- the site description, URL, layout template, etc. You can see how to create a blog at one site, Blogger.com, here and here.
This post gives tips for searching for blogs on a given topic.
If you create a blog, you might consider these usability tips from Jakob Nielsen. Also keep Jason Fried's comments about clear writing in mind when you compose blog entries.