Using the keyword "using" in C++ code

Is it a good practice to use the keyword "using" in C++ code? I have seen this very often: using std::string; using std::cin; usin std::cout; Why do we use it when we can simply include the header file (e.g. #include)? Please explain in detail.

The clause "using" is code requirement based practice it has nothing to do with the good practice. For better understanding I recommend you to go through following two links:

http://msdn.microsoft.com/en-us/library/aewtdfs3(v=vs.80).aspx

http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8l.doc%2Flanguage%2Fref%2Fusing_directive.htm