Synchronous and asynchronous classes difference

I'm reading "Foundation of Qt Development" book and I'm reading this line of text:
"QFtp and QHttp classes are asynchronous"

I know this classes are deprecated but the main reason why I create this thread is to figure out difference between Synchronous and asynchronous classes.

So,what's the difference?

The wording in the book is maybe not very precise. What it means is that both these classes perform their work in a non-blocking (asynchronous) manner. It means that by executing their functions you only schedule an operation to be performed -- when the function returns the operation is not yet complete. This is opposed to synchronous (blocking) calls, e.g. QFile::read() which block the flow until the operation is complete.

Thanks.
This is good simple explanation.