C++ using namespace declaration -

for a while I am using ...

  using namespace Do: std;   
 And recently I realized that  
  use namespace std; Can anyone explain to me what I was doing, and how is it different from the exact way of announcing the use of a specific name space?   
 
 
 The first version works because the compiler sees it  
  Namespace :: std; // Notice Location ^   
 This is simply telling the compiler to look for name-space  std  in global area.  
 Scope operator  ::  Anything on the left hand is similar to the global scope.