Different
Twitter is more of an up to the minute notification system of what you are doing at that exact moment. It is limited to a certain number of characters, I think 140 or less, so think of it as a text message. But, instead of sending the text message to 1 person, it is posted on twitter and people can elect to follow your messages and be alerted when you type a new one. It has caught on because people are following celebrities, athletes, etc. Companies are now trying to harness it to market their brand.
A blog is a web log, or think of it as articles that you post, but articles can be any length and it is more like a newspaper of what you are doing.
Clear as mud, yeah? ;D