From the National Review of Books:
The economic crisis in Greece is the most  important thing to have happened in Europe since the Balkan wars. That isn’t  because Greece is economically central to the European order: at barely 3 per  cent of Eurozone GDP, the Greek economy could vanish without trace and scarcely  be missed by anyone else. The dangers posed by the imminent Greek default are  all to do with how it happens.
I speak of the Greek default as a sure thing  because it is: the markets are pricing Greek government debt as if it has  already defaulted. This in itself is a huge deal, because the euro was built on  the assumption that no country in it would ever default, and as a result there  is no precedent and, more important still, no mechanism for what is about to  happen. The prospective default could come in any one of several different  flavours. From everybody’s perspective, the best of them would be what is known  as a ‘voluntary rollover’. In that scenario, the institutions that are owed  money by the Greek government will swallow heavily and, when their loan is due  to be repaid, will permit their borrowings to be rolled over into another long  loan. There is a gun-to-the-side-of-the-head aspect to this ‘voluntary’ deal,  since the relevant institutions are under enormous governmental pressure to  comply and are also faced with the fact that if they say no, they will have  triggered a proper default, which means their loans will plummet in value and  they’ll end up worse off. The deal on offer is: lend us more money, or lose most  of the money you’ve already lent.
This is, at the moment, the best-case  scenario and the current plan A. It reflects the failure of the original plan A,  which involved lending the government of George Papandreou €110 billion in May  last year in return for a promise to cut government spending and increase tax  revenue, both by unprecedented amounts. The joint European Central Bank-EU-IMF  loan was necessary because, in the aftermath of the financial crisis of 2008,  Greece was exposed as having an economy based on phoney data and cheap credit.  The cheap credit had now dried up, and Greece was faced by the simplest and  worst economic predicament of any government: it couldn’t pay its debts.