What is a flashback?

A flashback is a literary device used to plant a past scene into the present scene.

This can, essentially, be done in two main ways:

First, you can use a scene break and bring the reader to the past scene in question.

Second, if it's part of the dialogue, the speaker can tell about the past event.

I'm sure there are other ways I haven't thought of, or offshoots of these two, but for now let's limit the flashbacks to these.  We'll explore the use (or non-use) of flashbacks in the next few blogs.  In the meantime, please comment on whether or not you feel flashbacks are useful.  Also, what is your favorite flashback in a novel?