Resolution 

Image resolution is typically described in PPI, which refers to how many pixels are displayed per inch of an image. 

Higher resolutions mean that there more pixels per inch (PPI), resulting in more pixel information and creating a high-quality, crisp image. 

Images with lower resolutions have fewer pixels, and if those few pixels are too large (usually when an image is stretched), they can become visible like the image below.

When you change the resolution of an image, you are saying how many pixels you want to live in each inch of the image. For example, an image that has a resolution of 600 ppi will contain 600 pixels within each in of the image. 600 is a lot of pixels to live in just one inch, which is why 600ppi images will look very crisp and detailed. Now, compare that to an image with 72ppi, which has a lot fewer pixels per inch. As you’ve probably guessed, it won’t look nearly as sharp as the 600ppi image.

Resolution rule of thumb: When scanning or photographing, always try and capture the image at the largest resolution/quality.

Why?

It’s better to have more information than not enough! It’s much easier for image editing applications, like Photoshop, to discard any unwanted image information (reducing the size of an image) than it is to create new pixel information (enlarge an image).