The main benefit of using Ajax are mention bellow:
a. AJAX is a technique for creating fast and dynamic web pages.
b.AJAX allows to update parts of a web page, without reloading the whole page.
c. AJAX applications are browser- and platform-independent.

The parameters of open method of XMLHttpRequest object are :
method: The HTTP method used to open the connection,such as -GET,POST,PUT,HEAD,or PROPFIND
URL: The requested URL
asyncFlag: A boolean value indicating whether the call is asynchronous.
userName: The user name
password: The password

ResponseText holds the response body as a string.Which function is read-only.
ResponseXML holds the response body as XML.This function is also read-only.

0 uninitialized
1 loading
2 loaded
3 interactive
4 complete

The Status property indicates the combined values of the operation and error status code. The Status property for each Field can be used to determine why the Field was not added, modified, or deleted.