In C++, a namespace is a collection of related names or identifiers (functions, class, variables) which helps to separate these identifiers from similar identifiers in other namespaces or the global namespace. The std namespace is a namespace in the C++ standard library that contains many built-in classes and declared functions. The "using namespace std" statement is used to avoid having to write "std::" before every identifier from the std namespace that is used in the program. It brings all the identifiers of the std namespace into the current scope, making them available for use without the need for the "std::" prefix. However, it is considered a bad practice to use "using namespace std" in header files or large projects because it can lead to naming conflicts with other namespaces.