A blog is essentially a journal that is posted online. It is regularly updated and is usually informative or conversational. The most recent post is found first. Blogs can contain links to other websites or articles to further help present other resources. Blogs are considered a hobby but some people have managed to turn them into jobs. A blog is a medium of expression. It gives people the ability to express opinions and ideas over long distances instantly.
A Wiki is an application that can modify or even delete information or articles. Wikipedia was formed with the goal of allowing anyone free access to an encyclopedia. Wikis allow people to share information on the same medium. A wiki is essentially a chalk board that anyone can modify. It is a revolutionary way to educate people globally. It constantly changes as new information is discovered or changed. The information is not reviewed before it is posted. Majority rules.
Wikis are simple to edit and make significant changes to articles. Since the information is not reviewed before it is posted, anyone can post anything about anything. I believe this is both good and bad. It is good to allow free flowing conversations about knowledge and ideas. It is bad because if left unchecked, people can start believing things that aren't true. For the most part, it is a reliable source of information.