Measurements & numbers
Aspect ratio
The relationship between a sail's height and width: tall and narrow, or low and broad.
Aspect ratio describes how tall and narrow a sail is compared with its width. A high-aspect sail has a long luff and a short foot; a low-aspect sail is broader and shorter.
The number is a shape description, not a quality rating. Tall, narrow sails can be efficient upwind because they reduce some of the losses around the head and foot, and they work well in cleaner air aloft. Low-aspect sails put more area low down, are often easier to handle, and suit rigs or rules that limit mast height.
You see the difference immediately between a modern fractional mainsail and an older masthead genoa. The main is usually high-aspect, with height doing much of the work. A big overlapping genoa is lower-aspect: long foot, large overlap, plenty of power low and aft.