I don’t want to endorse his slant as an advocate, but Martin Feldstein gives a useful description of the “fiscal cliff" in his new Financial Times column "The US is unlikely to avoid ‘fiscal cliff.’” Here is his opening line:

The United States is rapidly approaching the “fiscal cliff,” a dangerous combination of increased taxes and decreased government spending scheduled for January 1 that would reduce the budget deficit by five percent of GDP between 2012 and 2013.

5% of GDP is a huge change.  

Quartz has made the fiscal cliff one of their “obsessions.” You can see Quartz’s articles on the fiscal cliff here.