When you take a photo with a digital camera, or scan a photo, you create an image that has a specific number of pixels on each side of the frame. For example, your digital camera may make an image that is 1024 pixels wide and 800 pixels high.
These two measurements, referred to as the pixel dimensions, have a direct correlation to the image's file size, and both are an indication of the amount of image data in an image. They represent the length and width of the image in pixels per inch.
Resolution is a measure of the number of pixels per linear unit of measure - for example, the number of pixels per inch (ppi). While a digital camera image contains a specific amount of image data, it doesn't have a specific physical output size or resolution. As you change the resolution of a file, its physical dimensions change, or as you change the width and height of an image the resolution changes.
You can see the relationship between image size and resolution using Photoshop Elements. In the image size dialog box (choose Image > Resize > Image Size). Deselect Resample Image, because you don't want to change the amount of image data in your photo. Then change the width or the height of the document or the resolution. Notice that as you change one value the other two values change.
You cannot improve (increase) on the resolution of the original image taken by the camera. This is a fallacy. You can increase the size of an image and the resulting image will get progressively more fuzzy.
To ensure you get the results you want in a print or when viewing on a screen, choose a camera that has the appropriate megapixel value to let you print or view at the size you want.