Let’s talk about resolution for a minute.
Every image on your computer is made of pixels–tiny little dots, each a different color, which, when viewed as a whole, make up an image. When you print that image out, the more pixels you can cram into the same amount of space (called resolution), the more detail your image has. Basically, the higher the resolution (measured in dots-per-inch or dpi), the more detail your image is capable of holding.
Here is a simulation of an image at 3 different resolutions, from highest on the left to lowest on the right:
Setting the resolution of an image in Photoshop (or other image editing program) has zero effect on the size the image appears on screen. Zip. Zilch. It’s only the pixel dimensions that matter when you’re looking at a picture on a monitor (unless, of course, you zoom in or out, but that’s cheating). On the web, your computer will show the image at the full size–each and every pixel. So if you want to change the size of the image on the screen, you need to change the actual pixel dimensions. So the conventional wisdom that says to set your images at 72dpi for the web actually has no effect at all! (By all means, test it out if you don’t believe me.)
On the other hand, when your image prints out, the printer isn’t concerned with the overall number of pixels in the image. It’s just concerned with how close together (ie. the resolution) it should print them. Of course, the more pixels you have to start with, the more you can raise the resolution and still have a high-quality print.