What is a wiki?
A wiki is a website or collection of web pages that can be created or added to by anyone with Internet access. It is a forum for the collaborative construction of knowledge - every educator's dream. This is made possible by online facilities such as PBWiki or Wikispaces.
The most well known wiki, of course, is Wikipedia, the free online encyclopedia that has been and continues to be added to by anyone in the world with Internet access. It is important to consider this when using Wikipedia for information gathering. This is not to say that all the information on Wikipedia is questionable but it does support the need to use various sources of information to cross-reference and verify the accuracy of information encountered.
What is a blog?
The word blog is short for web log which describes exactly what a blog is - a collection of online logs or journal entries to which others can add comment. Most often, it is a record of a person's learning or interest in a particular area, containing not only daily or frequent entries, but also links to supporting resources.
Some of the more popular blogs in the education arena are edublogs or blogger from Google.
Read this short article comparing blogs, wikis and google docs