Is it possible to make images and photos greater than 300 dpi in photoshop and how different would a 600 dpi be than a 300 dpi. Is so what would be maximum dpi and what type of situations and applications use dpi's greater than 300 dpi.
Finally where and how does one print images greater than 300 dpi - most printers dont.
First of all the big problem here is that you are mixing up PPI and DPI. PPI is pixels per inch and 300 PPI is the standard resolution setting for images printed in a magazine. It refers to the 160 lines per inch screens used for printing photographs when you publish them with a printing press. Computer monitors on the other hand array images at either 72 or 96 PPI. You are wrong about In Photoshop. In IMAGE>RESIZE you can change the Image Resolution to almost any number...certainly a resolution well beyond 300. But the ones that matter are 72, 96 and 300. Also setting an image at 600 PPI doesn't add anything to it. It just limits the final size you can print too.
DPI is dots per inch and that is a measure of the fineness of the image printed by a photo printer. My home printer has a DPI resolution that tops out at 6500 DPI. This means that the printer can lay down about 20 dots of ink per pixel. The pixel information sent to the printer tells it what the color and intensity of each pixel is. The more DPI the finer the more color accurate and denser the print. In fact a 300 DPI print would look kind of flat and faded. Most home printers print over 2000 DPI so you usually get clean images.
Another aspect of the relationship between PPI and DPI is that as you enlarge the physical size of the image for printing the PPI drops. In general as long as the PPI stays above 200 you will get great prints and by the way the difference between even using 200, 300 600 PPI is imperceptible in most prints.