What is a blog carnival?
A blog carnival is basically equivalent to a magazine for blogs. One blogger usually hosts it and gives a general topic/theme to other bloggers to write articles about. The bloggers submit their articles to the carnival host for approval. The host then publishes the carnival collection (usually a blog post with links to all the submissions). It’s a way to get more marketing for your blog if your article is included.
Here’s the official Wiki link: http://en.wikipedia.org/wiki/Blog_carnival