What is the 'debt ceiling'?
The above video gives a very good description of the debt ceiling. But some might put more emphasis on the current scale of US debt - which has edged up towards $17 trillion at an alarming rate - see debt clock here.
What brought the debt ceiling back into political play has been growing consensus that the debt is now unsustainable in the long term. The key question Democrats and Republicans battle over is what to do about this.
It is universally agreed debt default (see below) could have a potentially catastrophic impact on the world economy. This prospect eventually lead to a temporary deal between the warring parties.
For the US, however, none of the above options are feasible. It is literally too big to fail.