When you send an email you usually don't expect an immediate answer, but continue to do something else - that's like an asynchronous operation. In a day or two, you will hopefully recieve a reply, if it's important you will check your mailbox regularly, possibly sending a remainder too.
When you click on a link on a web page - you wait for the repsonse - that's like a synchronous operation.
Since we actually are talking about programming .Net has a lot of mechanisms to help you out - so some reading might help:
Here is a nice free e-book
] on threading and synchronization.
Checking out the classes in the System.Threading Namespace
] is useful too, you should also know about the IAsyncResult