In a Common Craft Blog Post, Lee Lefever analyzes the differences between Weblogs and Message Boards. Weblogs and Message Boards are different tools, and thus play different roles in online communities. The differences are as follows:

Locus of Control
Weblogs are written and controlled by individuals and small groups whereas message boards are group resources with control shared equally by all users.

Authoring of New Topics
The Locus of control defines who drives the content and who posts new topics. With Blogs new topics are started by the blogger or small group of bloggers who control the blog. In many message boards, any user can start a new topic.

Intent
With Blogs the Intention is often that trusted individuals provide content and information. With message Boards, the intention is often collaboration, accumulating decentralized expertise, and group decision-making.

Responses
Weblogs have comments. The author doesn't necessarily need to be involved, and there is no expectation that a reader will post a comment or that a discussion will ensue. Message Boards have replies. The expectation is that someone from the community will respond, and that there will be a discussion.

Tools
Weblogs have introduced new tools to increase readership and link different blogs together:
Trackbacks, RSS, Aggregation, Permalinks, Cross linking, etc.

Chronology
Weblogs are organized chronologically be the date of the original post, regardless of the date of the comments. Message Boards most often list threads in order of most recent replies, highlighting the relative importance of discussion.

Personal Connections
Readers of blogs form personal connections with the author(s). Because Message Boards have a more decentralized authorship, members have a broader view of the community.

Pollution Control
Because blog content is centrally authored and comments are secondary, spam or flaming is less of a problem than in Message Boards, where moderation is essential because replies are treated equally with original posts.

Content Buckets
Content organization in a weblog is usually done for archiving purposes, and often relies on categories or tags. In a Message Board, content threads or categories are usually presented upfront, offering multiple points of entry to the reader.