visitor is a browser in which website is open and this website make cookies on system browser. Cookies is a file that store in your system browser. And when the website open in same browser then GA know it this is not a new visitor. some time when cookies is not supported then it track through IP to know visitor is new or old. IP tracking is not a very good way to trace the visitor because many server and Dial-up connection change IP when they Reconnect. so GA prefer track by cookies first then if on some computer system cookies dose not allow then GA trace by IP to count the status of visitor. If some one know more please suggest