What is pixel aspect ratio?
When starting a new file in Photoshop, one is given the choice of selecting many things, among them is "Pixel Aspect Ratio".
What is this? What does it do?
Do I get smoother lines if I choose anything other than 'Square Pixels'? Will everyone see the same quality?
On a normal monitor imaging element is in a square matrix. We then call the aspect ratio of that pixel 1. Aspect ratio is just the width/height. A aspect ratio of 1 is a square and a aspect of 16/9 is elongated. In the case of monitors we have 2 separate aspect ratios the ratio of the monitor and the shape of each pixel, called pixel aspect ratio. These two are not to be confused together they are different things.
Image 1: Aspect ratio is defined as width divided by height
One some devices most notably old TV signals and some movie formats the pixel is not a square but rather somewhat elongated, wider than high. So by setting this value to other than 1 will result in Photoshop emulating such a screen by stretching your image accordingly.
Image 2: A pixel bobba fett by Shkvapper in a 1:1 pixel ratio and a 3:2 Ratio (or 1.5 expressed as one number)
Most users will never need this option for anything! So its safe to keep it at 1, unless you know you need it. Odds are you will never encounter a situation where you would ever need to change pixel aspect ratio.