I've noticed that the Twitter explosion has had an unfortunate affect on Old People (and some Young People): the term "tweet" is used interchangeably for long, short, analogue, and digital updates. At the risk of face-palming myself to the afterlife, I thought I might step in.
Tweet: A short 140 character update posted to one's Twitter account. Please note that the update must be 140 characters or less and must involve Twitter.
Microblog: A very short blog. When I was in college the in-class rule was a 200-word minimum. (200 words was also an idea goal, no one wants to read 5,000 words in a single entry.) If your contribution to the Internet is a few sentences at its maximum, without a specific structure (inverted pyramid or otherwise), you have a microblog. A microblog is quick, brief, and may include a photo, audio clip, or bit of text.
Blog: A singular post to a Web site. Like this one. An article on The Huffington Post, an update in a person's LiveJournal, is a blog. Again, if it's 200 words, it's no longer a microblog. A blog can be closed to membership, but it is not an e-mail.
How do you know what to call the item in your hands?
Did you post it (or are planning to) post the update to Twitter? You have a tweet.
Is the item longer than 140 characters and not going to Twitter? You have a microblog.
Is the item longer than 140 character and going to Twitter? You have a tweet. It needs to be edited or it won't post. (If you post from your phone Twitter will cut you off after 140 characters.)
Is the item 200 words or more? It's a blog. Post that baby online!
Is the item offline? It's not a tweet, dude. It's not a blog, or a microblog.
Is the item spoken? Then it's words. Coming out of your mouth. Don't get smart by calling your words "tweets."
Is the item an e-mail? It's an e-mail. An e-mail may be public discourse in the eye of the law in a libel suit, but it's not public enough to be considered a blog, even if the material is sent to 1,000 of your closest friends and family. (That is spam.) It's not a blog. Don't get smart with me, Mom.