Here’s a great post on Common Craft that explains the multiple differences between message boards and blogs. Scroll down to see Lee LeFever’s nifty side-by-side comparison chart.
1) Message boards are decentralized and group run; blogs are centralized and often published by one person.
2) Responses or “comments” are required on a message board in order to create a discussion; “comments” on a blog post are optional.
3) Chronology: blogs are always in reverse chronological order; topics and discussions on message boards can be organized and presented in different ways. Thanks to Shel Holtz for the pointer.