Que significa asincrono en ajax

http://stackoverflow.com/questions/3393751/what-does-asynchronous-means-in-ajax

Asynchronous means that the script will send a request to the server, and continue it's execution without waiting for the reply. As soon as reply is received a browser event is fired, which in turn allows the script to execute associated actions.

Ajax knows when to pull data from server, because you tell it when to do it.