What is argc, char* argv[1]?

[longberns]

Hello everyone!
I know this one might sound stupid but i must ask this one.

I have always known that after

 #include <iostream>
       using namespace std; 

comes,

 int main ()
{
bla bla bla

cin.get();
return 0;
}

But now i see this

include <iostream>

using namespace std;

int main(int argc, char* argv[])
{
    if (argc > 1)
        cout << "Hello " << argv[1];
    else
    cout << "Hello world!";

    cin.get();
    return 0;
}

1. What is this?
2. What does it do? When do we use it and why int main () suddently takes arguments?

Thanks for your expert opinions.

[JLBorges]

See http://en.cppreference.com/w/cpp/language/main_function

[longberns]

Thanks JLBorges