Why dont the whole world has a common currency. please explain the need of different currency.?

The world once had a single currency -- gold. If a country was importing more than it was exporting, gold would leave the country causing deflation until trade was again in balance. Changes in the domestic price level was what brought about a balance in trade. When each country has its own currency, changes in the exchange rate between currencies is what brings about a balance in trade. Each method has strengths and weaknesses.

because it started out like that in different areas, currency was different because the same currency couldnt spread to the whole world that long ago, and now theres so much, it would be too hard to make enough for all the countries, and then what would they do with the old currencies?

Your question implies the question, what is the optimal amount of money, and how or should it change to perhaps local circumstance? There is both a transactions demand and a financial demand for money. Because transactions and financial developments take place at different rates and different places, it would make sense to have a currency that is defined by the economic area which those transactions and investment takes place. For example, if we are on a island, and say we trade only 10% of the island economy with outsiders, we would not want the outside world determine the quantity or growth of money. Instead, we would want our money correspond to our own location. Also, we would want a looser money supply during times of unemployment and tighter during periods of expansion, assuming the usual sticky price Keysnesian model. This logic will suggest that currencies should cover economic areas that are similar to each other. The cost of multiple currencies is of course the difficulties in estimating the exchange rates and future prices of imports and exports. This generally reduces trade and hence output. If trade is widespread throughout the world, then it would make sense to have a common currency.

it has some valid economic basis ... it will be difficult to convert to one currency... but someday we will all have little carbonfiber cards (or similar) with our "value" added-to and subtracted from like credit cards... but until then, it also helps to keep the poor poor and the rich rich. dig it?