This is a popular, informal term denoting a beer
that is very out of the ordinary.
It could mean a beer that has an abnormally high alcoholic content for the style of beer it is, a very high level of bitterness, or other very unusual characteristics that put it outside the bounds of what we traditionally think of as "beer". This term is generally attributed to Todd and Jason Alström of beeradvocate.com.
- Extreme beer at Beer Advocate http://beeradvocate.com/articles/530
- Extreme beer defined http://www.wordspy.com/words/extremebeer.asp
- Article from IHT http://www.iht.com/articles/2008/01/10/style/09beer.php