Synchronize:
Synchronization idea is: all operations are done, just returned to the user. Such users online wait too long, giving the user a feeling stuck up (that is, system migration, click the migration, the interface is not moving, but the program is still running, stuck the feeling). In this case, the user interface can not be closed, if closed, namely the migration program was discontinued.
asynchronous:
The user request into the message queue, and feedback to the user, system migration process has started, you can close the browser. The program then go into the database and then slowly go. This is asynchronous. But users do not feel stuck, will tell you that your system has responded to the request. You can turn off the interface.
Synchronization, all operations are done before the results returned to the user. After that is finished database, the user, the user experience is not good.
Asynchronous operation done without waiting for all the requests to the appropriate user. That is, first the user request, and then slowly to write database, the user experience better.