Using the Euro Sign
The euro sign stands for the currency of the euro area, which covers most of the European Union. Its design, a rounded E crossed by two horizontal bars, was published by the European Commission in December 1996, a little over two years before the currency launched for accounting in 1999 and as notes and coins in 2002. Unicode encoded it at U+20AC, and because every operating system had to add it in a hurry, it now renders reliably on almost any device made this century.
Before or after the number?
There is no single European rule, which is why you will see both €20 and 20 € on the same holiday. The EU institutions write the sign before the figure with no space in English, Irish, Maltese and Dutch texts, and after the figure with a space in most other languages, including French, German, Spanish and Italian. When you are writing for a UK, Irish or international audience, €20 is the expected form; when you are quoting a German or French price, keep the order the source used.
Falling back to EUR
Banks, invoices and any table that might be imported into another program prefer the ISO 4217 code EUR, usually written before the amount with a space: EUR 4,500.00. The code survives plain-text emails, legacy ERP systems and fonts that only cover ASCII, and it removes any ambiguity between euro and other currencies that share a symbol. Use the sign in prose and on price tags; use the code in anything a machine will read.
Characters that get confused with it
The estimated symbol ℮ on food packaging looks like a small lowercase euro, but it certifies that the stated quantity meets EU averaging rules and has nothing to do with money. The Ukrainian letter Є and the Greek epsilon ε both resemble the sign in sans-serif fonts, and people occasionally paste them from a keyboard that lacks a real €. Searching a spreadsheet for prices will skip those rows, so check that the character you copied is U+20AC.
Typing it
On Windows the numeric keypad code Alt + 0128 comes from the Windows-1252 code page, which is why the number does not follow the usual Latin-1 pattern. UK and Irish keyboards also offer AltGr + 4, while many continental layouts put it on AltGr + E. On a Mac with a US layout, Option + Shift + 2 produces it, and phones hide it behind a long press on the dollar key. In HTML the named entity € is the readable option, though a UTF-8 page can simply contain the character.
In bios and captions people use € as a shorthand for money-related content, a shop link or a price list, sometimes wrapped in decorative characters from the aesthetic symbols page. It also appears alongside the pound sign and yen sign on this site if you need the whole set for a currency selector or a travel post.