Ok Todd and Tom, this is for you: Information about Resampling in Photoshop.
If you do not have enough pixel data to get the print size and resolution you need, then you’ll have to resample the image and Photoshop will create new pixels. This often results in poorer quality print results, so you need to judge which is the lesser of two evils: the loss in quality from reduced resolution or the loss in quality from resampling.
That’s where Resampling comes in. To cut to the bottom line, “resampling” means “making things up,” or perhaps “guessing.” Granted Photoshop makes educated guesses, but especially when adding pixels to increase resolution, that’s what they are, guesses. If the camera didn’t capture it, or the scanner didn’t scan it, the information to fill in those new pixels has to come from somewhere, even if it is out of thin air. Unlike on TV, crime detectives can’t extract details from film that were never captured in the first place.