En las peticiones XMLHttpRequest, tenemos varios status cuando el servidor responde, estos son los siguientes:

readyState The readyState code changes value from 0 to 4 during a request cycle:

0: not initialized.

1: connection established.

2: request received.

3: processing.

4: finished and response is ready.

status

200: "OK"

404: Page not found.

onreadystatechange: callback method assigned via this attribute

responseText: holds the response data as a string of characters.

responseXml: holds the response data as XML data.

Basicamente tenemos los estados 0 no inicializado, 1 conexión establecida, 2 petición recibida,  3 procesando y 4 finalizado y respuesta completa.