The difference between hemming a garment and creating a cuff on a garment seems small. A hem is purely for a practical purpose -- to keep the fabric from coming unraveled and worn out, and hems are folded inside the garment. A cuff, on the other hand, is folded up on the outside of the garment, so the 'wrong' side is facing out. This is often decorative.
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The Average Hem
When a seamstress creates a hem, she first folds up the edge of the material (wrong sides touching) and presses it. Then the fabric is folded up again, generally by about half an inch, pressed, and then sewn. This creates a neat, crisp hem. This is generally done in thread matching the garment. Sometimes, but not often, the hem is sewn after the first fold.
A Blind Hem
A blind hem reduces the lines of sewing that normal hemming creates, and is often used on dressy pants or skirts. This is because it is not as durable as a normal hem, and so shouldn't be used on an object that will receive heavy use. This can be done by hand, or it can use a special type of sewing machine stitch in combination with a special foot. While methods differ, the end product is a hem that only has visible stitches every half inch or so. These stitches should be very small, and they generally appear as tiny dots. When matching thread is used, this type of hemming is practically invisible.
Sewing a cuff permanently combines a cuff with the garment. The edge of the garment is folded up the desired amount -- how much fabric that is depends greatly on the desired size of the cuff -- and then sewn around the top, to be sure the cuff won't budge. Often the cuff is folded twice -- up once a small amount, and then up again, which tucks the unfinished edge out of sight and has the same effect as hemming a garment. Sometimes, though, cuffs are only sewn right at the seams, to give a more casual look to the garment, as though the object was not sewn.
These are cuffs that have not been sewn, and they are generally done by the person wearing the garment. This is done for stylistic purposes, generally, although sometimes it is done for comfort. Cuffing a shirt or a pair of jeans in cool weather can help a person cool off. In the 1950s, "greasers" cuffed their jeans to go with their look. Now the same style is used by people who consider themselves "rockabilly," harking back to the greaser look.