Unicode is a standard that facilitates electronic interchange of more than 100.000 characters by assigning a unique number to each character.
Here you can view them all.
The concrete visual representation is not defined by the standard and depends on the font you are using.
The numbers assigned to characters (called 'code points') are typically denoted by four- to six-digit hexadecimal numbers with the prefix 'U+'.
More information about Unicode can be found at unicode.org.
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