The notion of pixel aspect ratio
The pixel aspect ratio is the multiplication coefficient which links the pixel width to the pixel height. Depending on the video formats, the pixels may be either square or slightly rectangular.
Left: (pixel width) = (pixel height) x 2. The ratio is therefore « 2 ».
Right: (pixel width) = (pixel height). The ratio is « 1 » and the pixels are square.
The pixel aspect ratio has a value of 1.067 for the PAL format and a value of 0.9 for the NTSC format.
Avoid confusion between pixel aspect ratio and width/height ratio of a video (4/3, 16/9, …). The ratio is the width/height ratio of the pixels.
Using ratios needs a serious knowledge in the video environment. If you begin in that domain, we advise you to choose a simple 4/3 or 16/9 format with a square ratio (equal to 1). For example, 800 x 600 for 4/3 or 1600 x 900 for 16/9.