Why cin accepts string inputs after including String header

I am new to C++ programming. I was trying to take user inputs and put them in a variable and i was using cin . It was working for integer's and other's except strings. So, when i searched around i found that i have to include <string> header. i just want to understand, what changes occurred by including string header? i thought that a cin is overloaded by a function defined in string header. So, i started looking into String header, and i could not find cin overloaded or any function defined there at all. Can any one tell me how cin started accepting string inputs after including <string>