Apologies if the topic is not appropriate (economics newbie here) but I am curious as to who exactly would foot the bill if Greece defaults on the ~300 billion dollars it owes. It looks like most of the money is owed to EU, IMF and ECB but what does that really mean in layman terms? Does it mean that tax-payers in other (mostly EU) countries end up paying one way or the other.
You are correct. The majority of Greek sovereign debt is held by other EU sovereigns. So EU taxpayers end up footing the bill in the event of a Greek default. Usually, rather than outright defaulting (i.e. missing a repayment deadline), there will be some agreement to restructure the debt so that Greece gets more time or some other form of concession.
So the EU and Greece will both talk tough on TV to please their respective electorates, but both sides know that a genuine default would spell catastrophe for the entire European economy and so will do everything they can to avoid it.
We have first to be clear about what meaning do we give to the word "default". The other answer gave it a temporary meaning ("missing a payment deadline"). In such a case, any direct economic effect is bound to be small (expecially if the delay is short). But there may be indirect economic consequences through the effect on economic expectations.
Now let's give to the word "default" a heavy meaning: "non-payment ever of principal and interest", then there will be various direct and indirect effects:
The debt market loses Supply, reducing loan opportunities for other debtors. Creditors become more conservative and stringent towards prospective debtors, in view of the loss... in general the debt market suffers -and with it any beneficial economic activity that may need debt financing.
Creditors lose current income (the interest), and also become less wealthy. So their financial health worsens which affects directly their level of current engagement in economic activity, and also, their plunging into new economic activity, and so, the future. If the end-creditors are states, then their Government Budget suffers which may indeed lead also to higher taxation.
Consider now the following "economic fantasy" twist: Assume that a debtor state announces: "We cannot say when we will be able to start repaying the principal amount of the debt, but we will duly and fully pay the interest accrued in the meantime".... Is there a "bill to be footed" here? It would be very interesting to see what the consequences of such a situation would be. Unfortunately, in Social Sciences, there are all shorts of very interesting experiments that we cannot really execute, so we can only theorize about them, and hope for a "natural experiment" to emerge...