Hi,

The difference is that the sync. the send/receive operation block the
thread, where the Async method you pass a delegate that is a method which
will be called when there is data to be received for example.
In the MSDN Library you can find very explanatories descriptions of the
methods needed

When to use it ?
Depends of your escenario , frankly I have never used it , all the
communication system I've implemented are sync'ed. but if you have a app
that can receive/send data in a random or not in a constant way you could
use Async sockets.

cheers,