Both the EFSF and the Eurobond is every country in Europe teaming up to back their collective debt. When you loan these guys money, they're going to pay you back. Well, err, someone's going to pay you back. Italy's a bit short right now, so maybe Portugal can cover the payment. They're having riots against austerity? Well, how about France? They're losing their AAA rating and are cutting their budgets in a panic? OK, well, we all knew it would come to this, let's just get Germany to write you that check.
Eurobonds and the EFSF are essentially weaker German bonds. If the real, 100% pure, solid-gold German bonds are tanking, why in the world would you buy diluted German bonds?
It's Eurogeddon: Jeremy Warner over at the Telegraph has a must-read piece on the topic.