unicode

A mapping between each character used in writing (any kind of writing) and a number between 0 and 0x10FFFF = 1,114,111. Not all the numbers are used, and not all the writing systems are encoded yet.

For starters, all the numbers from 0xD800 to 0xDFFF (inclusive) are reserved for something called surrogates (which are used to encode the numbers >0xFFFF in pairs of 16-bit characters).

For lots more, see [1].