Sizes, resolution, zoom and dpi are often confused together.
Photoshop opens files at a zoom level that is attempting to show the entire image on your screen. If you have a really large image it might open up at 17% or 33% while smaller images will open at 100%. The zoom is based on pixels - 1 pixel will represent 1 pixel on your screen while 200% will have 1 pixel in the image will represent 4 pixels on the screen.
If you go to Image > Image Size you can see the resolution / dimensions of the image and rescale the image or change its dpi. If you play around in this window you might get a better understanding on all this. By unchecking the Resample Image box you are telling photoshop that you don't want to actually change any pixels in the image but you do want to change the "size" and dpi. Doing this allows you to resize a 10 inch image at 300 dpi to a 5 inch image at 200 dpi. However when you do this the actual size in pixels of the image will not change and will look exactly the same until you print it.
Abraham
