The number of pixels in a file determines its image resolution. If you have 72 pixels across a 1-inch horizontal line, your image resolution is 72 pixels per inch (ppi). If you have 300 pixels in 1 inch, your image resolution is 300 ppi.
Image resolution is critical to properly outputting files in these instances:
When you print images. If the image resolution is too low, the image prints poorly. If the resolution is too high, you waste time processing all the data that needs to be sent to your printer.
When you show images onscreen. Just like images have resolution inherent in their files, your computer monitor displays everything you see on it in a fixed resolution. Computer monitors display images at 72 ppi (or 85 or 96 ppi or higher). That’s all you get. What’s important to know is that you can always best view photos on your computer monitor at a 72-ppi image size in a 100-percent view.
Newer devices like smart phones and tablets take advantage of higher resolutions. You can find device display resolutions from 150 ppi to over 300 ppi on a variety of devices. When you design for different displays, it’s important to know the device display-resolution capabilities before you start working in Elements.
As an example, take a look at the following image. It is reduced to 50 percent and then at different zoom sizes. When the size changes, the resolution display on your monitor changes. When the size is 100 percent, you see the image exactly as it will print. The 100-percent size represents the image displayed on your monitor at 72 ppi, regardless of the resolution of the file.
This relationship between the image resolution and viewing the image at different zoom levels is an important concept to grasp. If you grab an image off the web and zoom in on it, you may see a view like the 800-percent view shown. If you acquire a digital camera image, you may need to zoom out to a 16-percent view to fit the entire image in the image window.
The reason that these displays vary so much is because of image resolution. That image you grabbed off a web page might be a 2-inch-square image at 72 ppi, and that digital camera image might be a 10-x-15-inch image at 240 ppi. To fill the entire window with the web image, you need to zoom in on the file.
When you zoom in, the resolution is lowered. The more you zoom, the lower the resolution on your monitor.
When you zoom into or out of an image, you change the resolution as it appears on your monitor. No resolution changes are made to the file. The image resolution remains the same until you use one of the Elements tools to reduce or increase the image resolution.
Image dimensions involve the physical size of your file. If the size is 4 x 5 inches, for example, the file can be any number of different resolution values. After the file is open in Elements, you can change the dimensions of the image, the resolution, or both.
When you change only the dimensions of an image (not the number of pixels it contains), an inverse relationship exists between the physical size of your image and the resolution. When image size is increased, resolution decreases. Conversely, when you raise resolution, you reduce image size.
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Source:http://www.dummies.com/how-to/content/image-resolution-and-dimensions-in-photoshop-eleme.html
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