Topic: [Info] About pixel aspect ratios,..
We differentiate between three types of aspect ratio:
Display Aspect Ratio (DAR): the aspect ratio of the image as displayed. ('4:3', '16:9', '16:11',...)
Storage Aspect Ratio (SAR): which is the ratio of pixel dimensions. ('720:480','720:576','1920:1080')
Pixel Aspect Ratio (PAR): which is ratio of the pixels themselfes. ('1:1', see below tables)
Here's how these three relate to each other: SAR × PAR = DAR
Meaning: If you multiply the shape of the pixels with their dimensions you get the display aspect ratio.
Since in video and image processing people can't make it simple there have been a lot of pixel aspect ratios definitions over the years, since the most common you will encounter are either the MPEG-4, ITU-R 601 or Generic defined values I will only list these:
and the most common as pixel aspect ratio: Square Pixel 1:1, which means that the DAR and the PAR are the same.
Is content is encoded with a pixel aspect ratio that is not 1:1 it's called anamorphic.
A good read about the topic is The Ultimate Guide to Anamorphic Widescreen DVD (forDummies!) by Bill Hunt from The Digital Bits.