To understand what all is Sub prime crisis first have to understand what is sub prime loan.... 

Since the risk of default on such loans was higher, the interest rate charged on sub-prime loans was typically about two percentage points higher than the interest on prime loans. This, of course, only added to the risk of sub-prime borrowers defaulting. The repayment capacity of sub-prime borrowers was in any case doubtful. The higher interest rate additionally meant substantially higher EMIs than for prime borrowers, further raising the risk of default.

Further, lenders devised new instruments to reach out to more sub-prime borrowers. Being flush with funds they were willing to compromise on prudential norms. In one of the instruments they devised , they asked the borrowers to pay only the interest portion to begin with. The repayment of the principal portion was to start after two years. How did it turn into crisis? The housing boom in the US started petering out in 2007. One major reason was that the boom had led to a massive increase in the supply of housing. Thus house prices started falling. This increased the default rate among subprime borrowers, many of whom were no longer able or willing to pay through their nose to buy a house that was declining in value.

Since in home loans in the US, the collateral is typically the home being bought, this increased the supply of houses for sale while lowering the demand, thereby lowering prices even further and setting off a vicious cycle. That this coincided with a slowdown in the US economy only made matters worse. Estimates are that US housing prices have dropped by almost 50% from their peak in 2006 in some cases. The declining value of the collateral means that lenders are left with less than the value of their loans and hence have to book losses. How did it became a systemic crisis? One major reason is that the original lenders had further sold their portfolios to other players in the market. There were also complex derivatives developed based on the loan portfolios, which were also sold to other players, some of whom then sold it on further and so on.

As a result, nobody is absolutely sure what the size of the losses will be when the dust ultimately settles down. Nobody is also very sure exactly who will take how much of a hit. It is also important to realize that the crisis has not affected only reckless lenders. For instance, Freddie Mac and Fannie Mae, which owned or guaranteed more than half of the roughly $12 trillion outstanding in home mortgages in the US, were widely perceived as being more prudent than most in their lending practices. However, the housing bust meant that they too had to suffer losses — $14 billion combined in the last four quarters - because of declining prices for their collateral and increased default rates.

The forced retreat of these two mortgage giants from the market, of course, only adds to every other player's woes.   What has been the impact of the crisis?

Global banks and brokerages have had to write off an estimated $512 billion in sub-prime losses so far, with the largest hits taken by Citigroup ($55.1 bn) and Merrill Lynch ($52.2 bn). A little more than half of these losses, or $260 bn, have been suffered by US-based firms, $227 billion by European firms and a relatively modest $24 bn by Asian ones. Despite efforts by the US Federal Reserve to offer some financial assistance to the beleaguered financial sector, it has led to the collapse of Bear Sterns, one of the world's largest investment banks and securities trading firm. Bear Sterns was bought out by JP Morgan Chase with some help from the Fed.

The crisis has also seen Lehman Brothers - the fourth largest investment bank in the US - file for bankruptcy. Merrill Lynch has been bought out by Bank of America. Freddie Mac and Fannie Mae have effectively been nationalized to prevent them from going under.

How is rest of the world affected?Apart from the fact that banks based in other parts of the world also suffered losses from the subprime market, there are two major ways in which the effect is felt across the globe. First, the US is the biggest borrower in the world since most countries hold their foreign exchange reserves in dollars and invest them in US securities.

Thus, any crisis in the US has a direct bearing on other countries, particularly those with large reserves like Japan, China and - to a lesser extent - India. Also, since global equity markets are closely interlinked through institutional investors, any crisis affecting these investors sees a contagion effect throughout the world.