Video jargon can quickly get complicated. But the basics of video are comparatively simple and easily learned, and the rest can follow from that.
This the picture’s proportions as width versus height. DSLR sensors have a 3:2 ratio, so that photographs are 3 units wide to 2 units high. Most compact camera sensors have a slightly squarer 4:3 aspect ratio. It doesn’t matter what the units are – the ratio stays the same, so a photo could measure 3 inches by 2 inches or 6 meters by 4 meters and still have the same 3:2 aspect ratio. You can shoot in different aspect ratios by cropping the sensor area. HD video is shot in a wider 16:9 ratio.
The aspect ratio of an image is the ratio of its width to its height. The larger the ratio, the ‘wider’ the image; the smaller the ratio, the ‘squarer’ the image.