A blog is like a diary or journal, with entries that are arranged in the order in which they were created. A wiki, on the other hand, contains shared content that doesn't appear in chronological order. The type of information you want to put on your site will help determine whether it appears in a wiki or in a blog.
Blogs are best for news. Entries appear in the order in which they were created, with the most recent entry appearing first. Only the most current entries appear on the main page of the blog. Older entries are still available, and can be found and viewed by searching or navigating using the date controls.
Wikis are best for group collaboration and research. Wiki pages can easily link to each other. When you create new wiki pages, they are instantly available for searching and for other pages to link to.
Mac OS X Server includes both a group wiki and a group blog. Users can also publish their own personal blog using web services associated with their server account.