photo by liz west
The simple answer is: if you know what you're doing use tiff (uncompressed). If you're not sure what you're doing you'll probably want to use jpg (compressed).
The best answer is: ask the print lab where you're having the prints made what they recommend. If you're just going it yourself you might want to consider using tiff files. If you don't understand
Prints and color are actually very deep subjects. Don't worry though, the best bet is to really ask an expert. Making good photographic prints is truly an artform in and of itself.