What they are useful for: Blogs can be used for a variety of different things, such as news, personal interests, and journaling. They can be used for anything and everything you want since they website is owned and operated by you!
How blogs are a distinctive type of writing: They are extremely "hypermedia". That is to say, they are comprised of text, pictures, videos, comments and thoughts on media throughout the interweb, and just about anything you can embed into your blog.
Blogs can be an effective extension of classroom learning by providing opportunities for students to learn from an outside source, but more importantly from each other.