The term “aspect ratio” with DVDs is usually applied to the ratio of width to height of the video. Such as 4:3 or 16:9. But there is also the pixel aspect ratio. YouTube, for instance, will only accept videos with a 1:1 PAR, i.e. “square pixels”. Everything Blu-ray has square pixels. DVDs, however, do not.
DVDs are 720 pixels wide by 480 pixels tall, but depending on its two modes, the width is either shrunk or expanded. (This is for NTSC encoding, as opposed to PAL.) Depending on whether the video is 16:9 or 4:3, in square pixels the real width and height is either 854×480 or 640×480.
So NTSC is North America and Japan and a few other places. It’s just a kind of encoding of the original video. PAL is different regions of the world. A PAL DVD will be 720×576, and its pixels will be shrunk width-wise beyond both 4:3 and 16:9. If it’s set as widescreen (16:9), then to get square pixels you would change the width to 1024×576. If it’s fullscreen (4:3) you would change the width to 768×576.