Answer
Assume two pages A and B where page B is from another domain both are in the same < iframe > element. You can still send message from Page A to Page B and vice versa. That's called cross-domain messaging.

Answer
Assume two pages A and B from the same directory on the server and both appears in an < iframe > element. Now you can send text messages from page A to page B and make use of those messages in page B. That’s cross-window messaging.