What the yen sign means
The ¥ sign stands for the Japanese yen (JPY) and is also used for the Chinese yuan renminbi (CNY). It is a capital Y with one or two horizontal bars; both versions are the same character, and the font decides how many bars you see.
In English, ¥ goes before the number with no space: ¥500, ¥12,000. Yen prices rarely need decimals, because the smallest coin in use is one yen. In Japan the currency is called en, written 円, and prices often put 円 after the number: 500円.
In China, everyday prices use 元 (yuán) after the number, as in 100元, while the official name of the currency is renminbi (RMB). When both currencies could appear, write JPY and CNY, or CN¥ for the yuan, to avoid confusion.
Why ¥ appears instead of a backslash
If you use a Japanese version of Windows, you may see file paths like C:¥Users¥Documents. Older Japanese character sets put the yen sign where ASCII has the backslash, and Japanese fonts still draw that code as ¥. The character underneath is a normal backslash, so the paths work exactly the same. Korean systems do the same thing with the won sign ₩.
How to type ¥
On a Mac with the US layout, press Option + Y. On Windows, hold Alt and type 0165 (or 157) on the number pad. Japanese keyboards have a dedicated ¥ key, and with a Japanese input method you can type "en" and convert it to 円.
On iPhone, ¥ is on the #+= symbols page; on Android, Gboard has it on the second symbols page. In HTML, use ¥. The fullwidth ￥ matches the width of Japanese and Chinese characters. For more characters from both languages, see Japanese symbols and Chinese symbols, and for every money sign, currency symbols.