They differ in depth. A blog post is detailed and a microblog post is short.
Tweeter is a microblog, in 140 characters you can share a lot of useful (or pointless) stuff, from exactly what you are doing right now, to a link which explains the entire history of World War 2.
Another great example of a microblog would be one related to books. You could share quotes, passages, summary and personal feelings about a book. Tumblr is famous for this format
I have also seen bloggers combining the two perfectly, they write their long detailed posts, but also share content they find elsewhere, followed by a short description of why they thought of sharing that with their audience