When people talk about viral videos, memes, and such, they mean videos or more generally ideas that spread (especially online) as if they have a life of their own.  This video by Anthropology professor Mike Wesch was originally put online in 2007 and now in 2009 has be viewed almost 10,500,000 thousand times.  It captured what a lot of people were thinking and set it to a catchy tune.  This is also an example of a screencast used not to teach a “how-to” concept, but rather to explore more abstract issues.