Unicode is a character encoding standard that includes a much larger set of characters than ASCII, which allows it to represent most of the world's writing systems. This is essential for documents that require the use of characters from multiple languages that are not all covered by the ASCII character set. Binary is a number system that computers use to represent all types of data, but it is not a character encoding standard. Hexadecimal is often used as a human-friendly representation of binary data, especially in contexts such as color codes in HTML, but is not used for text character encoding. ASCII is limited to 128 characters, which is insufficient for encoding the characters of multiple languages.