Here is a table of who owes who in Europe:
The Greeks are eventually not going to pay their debts in full - given their size of debt which is 160% the size of their GDP. They have a govt budget deficit of 13% of GDP. Many pundits are saying Greece will soon default on their debt[BBC Report] by rescheduling it because they can no longer service the debt that is due. Default fears has cause markets to fall as it will most certainly revive the panic of May 2010 when the situation reach crisis proportions. However, what is likely to happen is they will kick the can further down the road to deal with a Greek default at a better time on the assumption that the global economy will eventually recover. Allowing Greece to default now creates a lose-lose situation immediately - the Greek economy will just collapse taking many of the the European banks down with it.
The problems in Europe just shows how fragile the global economy is. Right now central banks have pumped money and keep their fingers cross that the whole system can reach a better state so that more permanent solutions can be found for many of the problems. However, if something derails the economy - terrorist attacks, natural disasters, market disruptions - these fissures in the global economy will blow open and the system can sink very quickly.