HTTP is used for sending requests from a Web client, usually a browser, to a Web server. HTTP is also used to return the Web content from the server back to the client.
HTTPS handles secure communication between a client and server. Typically, it handles credit card transactions and other sensitive data.
The typical request and response mechanism is synchronous. In synchronous messaging, the replay engine blocks script execution until the server sends its response. In asynchronous mode, the replay engine executes the script without waiting for server's response for previous messages.
If you are working with HTTP or HTTPS transport, you can use asynchronous calls in conjunction with WS-Addressing. For details, see WS-Addressing.