Also, Americans use at least 2 bedsheets, and use of 3 is becoming more popular. The minimum is a fitted bottom sheet that has elastic edges and fits around the mattress, plus a flat top sheet that has no elastic, and which is usually tucked in at the bottom to keep it in place on the bed and to keep your feet from drafts. The comforter (or duvet if you prefer) goes on top of the flat sheet. People who use 3 sheets use two flat top sheets, with a light blanket sandwiched between them. This style is known as "double-sheeting" and it originated at hotels, where it was designed to reduce the need to wash the blankets (which don't hold up to hot water washing as well as sheets do.)
Here's the key to the different style: washing. You use a duvet with a removable washable cover that goes next to your skin. Our duvet-equivalent (the comforter) does not normally have a removable washable cover, so we use the top sheet to keep the comforter away from our skin and thus reduce the need to wash it too often. Just as with washing a duvet, washing a comforter normally cannot be done at home; it requires a large-capacity commercial machine and a trip to the laundrette.
A pillow sham is always used when the bed is made up with a comforter, rather than the more old-fashioned "bedspread". (Bedspreads are almost never filled with anything, and are much larger than a duvet; they reach the floor on both sides and the foot of the bed, and have extra allowance at the top for covering the pillows with a pleat underneath them. If you use a bedspread you don't have to use shams or a bed-skirt, though now most people use shams anyway, as it's easier to make the bed that way.) Shams are decorative pillow covers, and it is a pain to remove them to sleep, so many people keep two sets of pillows for the bed; one for sleeping on and one for show that is removed during the night.
This is traditional bedspread style such as your granny might prefer (I know that this style was also used in the UK pre-war; my mother always made up beds this way, and she left England in 1951. Getting the pillow-roll tucked just right on a large bed requires a lot of bending and is tedious.):
The compromise version of the bedspread style uses the big floor-length spread, but shams as well. This also shows the alternate style of bedspread that has a seam around the bed perimeter and a ruffled drop instead of the bell-pleat at the corners:
The most popular style now is a comforter used with shams and a bedskirt, though at home most people would only use two shams, and not the pile of extra shams and throw pillows that this advert shows. Note the folded-back cream topsheet that is visible in the photo:
PS: We don't often use the term "pillow cover". The most common term for the fabric covering for a pillow that is meant to be slept on is a
pillowcase, or sometimes
pillow slip. Shams are not normally slept on directly, as oil from your hair would stain them.
Some Americans will use down comforters with a duvet cover on, but these are premium expensive products, and not usually sold in a set. If you buy a down comforter you are buying the comforter only. You might be able to buy the duvet cover as part of a set including a bedskirt and shams, but it would be fairly unusual to find a set like that.
A "sham" is a decorative cover that goes over the pillow and pillowcase, usually made to match the comforter. Our pillows don't usually have a cover like a duvet, we just use a pillowcase.
