There are numerous posts and document on conceptual explanation of one of the central concepts in Twisted framework - deferred.
The book on Twisted network programming provides an analogy of deferreds as buzzers which are handed to a visitor by a restaurant owner. This buzzer notifies the visitor that the table is ready and he could set aside what ever he has been doing and can come over to occupy the table meant for him.
Others identify deferreds as a place holder or a promise that is yet to be fulfilled. We could attach other actions that should follow when the promise is fulfilled or breached. These actions are like callback chains that would be triggered when a deferred fires.
Deferreds allows you to create followup action for something that will take some time to get fulfilled. This in turn relieves twisted to attend to other tasks and come back to execute follow-up actions when the condition is completed.
I will keep myself to code commentary and current behavior of deferreds.