Asynchronous literally means not synchronous. We are making HTTP requests which are asynchronous, means we are not waiting for the server response. We continue with other block and respond to the server response when we received.
The term Non-Blocking is widely used with IO. For example non-blocking read/write calls return with whatever they can do and expect caller to execute the call again. Read will wait until it has some data and put calling thread to sleep.
