The people at the internet have created these videos to tell us in plain English (albeit with a fairly strong American accent) what a wiki is and what a blog is.

Blogs:

Wikis:

The differences

Even though the two are both ways to publish information online, the main differences lie in their organisation.

A blog is organised as one long page of information whereas a wiki is a series of pages that inter-connect. As a result,

You would use a blog as a journal online where your most recent thoughts are at the top (like this blog).

You would use a wiki to display equally important information linked to from a central page (like Wikipedia).

Blog-like characteristics in wikis and wiki-like characteristics in blogs

While these differences are characteristics of the two applications, many blogs (such as this one) have an index along the side that keeps track of the topics covered. In the same vein, many wikis will have a box that shows the most recent contributions. So, yeah.

Hope this clears things up.