Async postback Trigger in Ajax

hi,

synchronous postback: Complete page goes to server and back with the result.
 
asynchronous postback: Partial page/Part of the web goes to the server(concern part) and back with the result.
 
Example:
 
If you have 2 drop-downs in your page 1 for country and other for city, binding of the cities in the city drop-down totally depend on the country, instead of sending whole page to the server just send/update those drop-downs to server which are our concern(other page parts will be static so it will remains the same at client sideand no need to send them to server) that will increase performance of our web applications.