A couple of months ago a WCMS sales guy said to me that when hears the words "we are looking for blogs, wikis, etc." from a customer it is a clear indication that the customer really doesn't know what he is talking about or (at least) doesn't have a clear vision of goals for Web 2.0.
I too am suspicious (and a little surprised) when I hear these terms together because, other than the fact that they are relatively new to the "enterprise," blogs and wikis have little to do with each other. Bob Doyle wrote a very good article differentiating these technologies way back in 2006 (When to Wiki, When to Blog - read the article).
A blog is a publishing system and a wiki is a collaboration tool. A blog author writes articles (posts) which reflect an idea or an observation at a point of time. You don't typically update a blog entry unless you see a typo that annoys too much to ignore (like misspelling your name - as on of my recent posts). Comments provide a forum for a dialog around the topic. These comments may appear within the context of the blog site or somewhere else as in the case of friendfeed but they are a conversation around the article, not the article itself. Occasionally the blog author will highlight a comment by updating the blog with a reference but this is the exception not the rule. If the author changes his mind, he will write another post rather than update the original. To learn from blogs you read lots of posts and piece together a consistent understanding that works for you.
A wiki is a tool to collaboratively build a comprehensive informational resource. Rather than blog posts that a single author publishes to an audience, a wiki page allows a group of people to jointly define a topic, establish a policy, or create some other information resource that needs to be updated over time. Companies that use a wiki (rather than a WCMS) as their intranet have come to the conclusion that potentially anyone in the company could correct or otherwise improve the information there. If these contributions are wrong, their updates can be corrected or rolled back.
WCMS can serve both of these publishing and information management purposes. For example, a typical implementation "corporate brochure" of a CMS will publish "point-in-time" articles (e.g. press releases) and manage fixed pages (e.g. "about us"). If you need to do both with one tool (and want the option to strictly control contribution), you probably need a WCMS. If you need to do one of these things but not the other, you might be in the market for a blog or a wiki but not "blogs, wikis, etc."