Dennis Kennedy is Explaining Blogs and RSS: A Primer, and I figure I can take a shot too. As Dennis says in his comments, the difficulty lies in describing blogs in words. Doing a live demonstration is frequently much easier because you can show people what you mean, rather than tell them. I like Dennis' attempt to focus on the effect, rather than the technology for blogs. With RSS (web feeds), there might be a need to at least mention technology.
Blogs are places where you can read the regular writings of friends, colleagues, clients or industry experts. You'll find a variety of writing styles, from journalistic to informal. The articles tend to be less formal than journalistic or academic writing, and you will find writers who post things every day and those who post weekly. The ideas for articles frequently come from articles that have been posted elsewhere on blogs (such as this one), or in the newspaper or in current events. Articles range from a few sentences to lengthy discussions, though most tend to be shorter.
Over time, the experience of reading a blog gives you a deeper sense of the author and the things that engage them. Reading several blogs that focus on the same topic (i.e., knowledge management or librarianship) can give you a sense not only of what each author is thinking but of how the community or discipline is evolving its thinking on the topic or in reaction to a news event.
Beyond the reading experience, many blogs give you the opportunity to participate in the conversation by writing to the authors or posting comments directly on their blog. This enhances your ties to that blog, since you begin to have a deeper interest in the articles.
But doesn't trying to keep track of all those blogs become a time sink? This is where the idea of syndication comes into play (web feeds, RSS, Atom). Just like the Associated Press or Reuters, most bloggers provide a "newswire" of their articles that can be pulled together in your very own newspaper. These electronic newspapers aggregate across all the blogs you read and present the only new articles for you in a single location. This makes it very easy to keep track of what your friends, colleagues and industry experts are saying. These tools are called aggregators or feed readers and can be anything from web-based tools to stand alone applications to applications that work within existing software. Here is one starting point, NewsReaders.com.
Feel free to compare to Dennis' definitions or even to your own.