Came across the character 帷 for the first time the other day, in the word 窗帷.
Oxford, KEY and Adstrotrans define 帷, simply, as:
curtain
ABC and A Chinese-English Dictionary define it as a:
bed-curtain
which doesn't really mean much to me.
What's the difference between 帷 and 帘?
I'll assume you mean 帘 as in the simplified Chinese for 簾. In which case, strictly speaking:
帘 / 簾 is a cover for windows woven from bamboo slips (hence the traditional top radical) or fabric
帷 is curtains, in the sense of sides a tent woven from fabrics.
In practice, they both mean some sort of fabric material that obscures sight, and therefore usages have become mixed up. In this case, the 窗 in the word 窗帷 specifies its a window curtain, making it the same as 帘 / 簾.
Traditionally, 帘 is actually a different character from 簾. It meant "a banner (of an inn or tavern)", and was flown outside such establishments.
《唐·鄭谷·旅寓洛南村舍》
村落清明近，秋千稚女誇。春陰妨柳絮，月黑見梨花。
白鳥窺魚網，青帘認酒家。幽棲雖自適，交友在京華。
Because 帘 also happens to be the simplified Chinese character for 簾, this difference is not noticeable in a majority of online resources anymore.
Historically, 簾 is what court officials use while 帷 is what common folks use. This is Ancient China historical.
《禮緯》 天子外屛，諸侯內屛，大夫以簾，士以帷
In fact, though there is difference between 帏 and 帘 in there orgin meanings as described above, but 窗帏, 窗帷, 窗帘 are the same thing. Usually people use 窗帘, and 窗帏 more seen in ancient writing or arts. You can use 窗帏 in writing, but if you spoke of 窗帏, others may not understand what you are speaking of at once.
I am a Chinese. I think both of them mean 'curtain'. Below are some differences I could think of:
(1)
帷 is rarely used nowadays. Using 帷 in Chinese sounds like using Shakespearean language in English. A writer may use this character on purpose to make his writing more 'classical', but using this character in everyday language is very weird...
帘 is commonly-used now.
(2) 帷 is larger, thicker and more luxurious than 帘 - think of the stage curtain in a theatre or the curtain in a palace. 帘 is more appropriate when you want to describe the curtain in your bedroom.
