Debt Crisis Greece and America?

ok...so im a dumb dumb...i have a hard time understanding economy, and money circulation...but i do know America is in massive debt, along with Greece (obviously) and im sure many other countries as well...but my question is, to who? how? who does America own money to? who does Greece own money to? i mean, if theres a debt crises someone has the money right? i dont understand where taht money is, where did it go? and how does that effect the people, why do they begin to riot? how are the people of a nation directly affected, to the point of mass violence and protests because theyre government has no money? can someone please explain this to me in the simplest possible way, i just dont understand =/ thanks...