Very often your photo editing software will be able to straighten and crop images at the same time, but there are still times you might want to treat them as two separate tasks.
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.
Cropping a photo is usually seen as a creative choice, but in the real world it’s not always quite that simple. Very often, you need to produce an image to fit a specific display size or aspect ratio, and you won’t know what that is until the time comes.