Overexpose backround for portraits

I am mostly a landscape guy so this may be a stupid question. But, I have seen some portrait photos, especially some wedding shots where the backround is slightly overexposed and the subject shot is perfectly lit. How is this done? Is this done in layers in photoshop or can this be done in camera?

I would imagine that if you either properly expose or slightly overexpose your ambient and then let the flash determine the correct exposure for your subject that would do it. I don't think it's any more complicated than that.

I think if you over expose your ambient and then hit it with flash your subject will be over exposed too. You have to add light to the background or reduce the light on the subject.

Put the subject in the shades, properly expose the subject, if the BG is under the sun, it will be overexposed. Or simply turn the subject around with the sun behind, meter off the subject's face, the BG will overexposed also.

It's lot easier in studio, since we can light BG separately from the main subject.