What happens to a country with lot of debt? As far as I know, debt is a way to get hands on extra money which will burden the future governments.


What if every successive government keeps adding to the national debt? 
Isn't it like free money? You can win by developing the country and keep adding to the debt as well... maybe forever? 
The US has a debt of $18 trillion. How has it affected United States? Why should the government even bother to repay?
If a government owes money to domestic or foreign entities and does not repay that debt then the government's credibility — just like any other borrowing entity's credibility — as a reliable economic entity takes a hit. As a consequence when it wants to borrow more money in the future, the lending entities either charge a higher interest rate to cover the risk or refuse to lend to avoid the risk altogether. 

Here's the only difference — governments can print their own currency unlike other entities. But too much currency in circulation will cause inflation, which is bad for the economy. So it is in the government's own interests to repay its debt, as it is to borrow money in the first place.