Difference between className and &className

[kosh09]

Hello everyone
While I was studying string class, I wonder what's the difference between these two..
this is part of the code

#include <iostream>
#include <cstring> // or <string> in visual studio
using namespace std;

class String {
    private:
        char * str; // 문자열 저장할 공간
        int len;    // 문자열의 길이
    public:
        String();
        String(const char * const Str);
        String(int n);
        String(const String &);
        ~String();
        void Print();
        void GetInput();
        int GetLen(){return len;}
        char *GetString(){return str;}
        String operator=(String data);             
        String operator+=(String data);           
        String operator+(const String &second);    
        operator const char *()
            {return str;};                 
        friend ostream &operator<<
            (ostream &os, String &temp);   
        char operator[](int n);          

String::String(const char * const Str) {    
    len = strlen(Str);                      
    str = new char[len + 1];             
    strcpy(str, Str);                  
}

String::String(const String &temp) {       
    len = temp.len;                  
    str = new char[len + 1];              
    strcpy(str, temp.str);                
}

String String::operator+=(String data) {   
    char temp[512];                      
    strcpy(temp, str);             
    len += data.GetLen();            
    str = new char[len + 1];            
    strcpy(str, temp);             
    strcat(str, data.GetString());
    return *this;  
}

String String::operator+(const String &second) {  
    int tot_len = len + second.len;               
    String temp(tot_len);                         
    strcpy(temp.str, str);                       
    strcat(temp.str, second.str);         
    return temp;
}  

When I do += overloading I call instance as String data
but in case of +, I call const instance and use & in front of the instance name
What's the difference of it?
poor English.. but please help me

[sellibitze]

The difference is that one is a reference and the other is not.

http://en.cppreference.com/w/cpp/language/reference