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Ros and std are C++ namespaces. These namespaces are comprised of many different files.

std: The standard C++ library. When programers use this line:

using namespace std;

They are stating that they are using the standard namespace (allows you to use the variables without putting std:: in front of them).

For example:

std::cout<<"Hello world!";

becomes:

cout<<"Hello world!";

When mixing namespaces, it is of bad practice to do the following:

using namespace std; 
using namespace ros;

Reason being, you may overload some methods you don't intend to. You can read more about this here.