Trade deficit is when one country is importing more compared to its exports. Exporting more than imports means one is earning more (because exports are all about selling) than spending (because imports are all about buying). Now for this to happen one has to then naturally get that additional money from somewhere. For instance if one country exports 1 billion dollar worth of goods, but imports 5 billion dollar of goods, then the difference of 4 billion dollars should have some source. So in case of US it has been the constant borrowing of funds by the US government by issuing its treasury bonds which are accepted worldwide. Then these funds generated by the borrowings (by selling treasury bonds) are used to fund the trade deficit. Have written a detailed article on this at http://www.hitxp.com/articles/economics/us-financial-debt-ceiling-crisis-simplified-summary-prevent-default/

Also can you explain a bit further about Trade deficit and how it got started at first place… 
I mean till US was exporting its OIL there was no(zero) trade deficit at all?? and since OIL reserves were found in other countries, trade deficit in US started ?? and to which countries do they owe these trade deficits or Debts and which currency they need to pay these debts ???