Different from other webpages:
Probably because blogging has it's roots in personal journalling, most blogs retain an informality of language and topic. This can be a positive trait even for an organizational blog since it reflects an honesty and earnestness that some users can strongly relate too. In most cases blog posts measure incremental progress or current information, not long polished essays. That means a blog is not generally a substitute for other pages online but can provide supplementary details and facts for people interersted in digging deeper and keeping up to date on a particular subject.
Most blogs allow comments. Indeed this is something we are trying to encourage on our blogs, The ease of two way communication or even the potential for two way communication is a big difference between blogs and other pages.
Blogs have a tendency to be focussed on a niche. Although there are notable exceptions of megablogs like Slashdot and Boing Boing most of the growth in blogs now is happening in niche blogs serving a dedicated audience with focussed information. It makes sense for most organizations (or anyone else) to focus on topics where they are most credible. That won't bring you billions of visitors but it will grow your audience over the long term.
Author's are credited. By that I mean each post is signed. Although some other online writing is credited it is not standardized in the way a blog post is. Knowing who is behind the words has a subtle but profound effect on the tone of an article. Instead of reading a post from the "monolithic Walker Art Center" you are instead reading a post from "Paul Schmelzer". The humanizing effect this has on the writing and the sense of ownership it instills in the authors is very different from standard web writing.
Several important web services have grown up around blogs. Services like Ping-o-Matic help small publishers get the word out. Ping services focus on new content, that means blogs that post more to attract attention. Secondary services like Technorati add filters to ping services intended to sort out quality or authorative posts.
Blog posts are archived. Most of the time when you change a webpage the old page does not exist as a public archive. There are things like Google cache, and the Internet Archive but to the general public the page is gone. That is not so with a blog. Old posts get pushed down, and are archived when they go off the page. Old posts are treated as less relevent by most search engines but they are still available.
How is it different from email newsletters. Getting the word out about programs via a blog has many of the benefits of an email newsletter without the associated spammyness.
In otherwords, people seek out your blog and it's RSS feed. They subscribe to the RSS feed which means they are interested in checking back later. Then similar to an email newsletter they are notified about changes. Unlike an email the user decides when to download it and when to be notified of updates.