The UNICODE Standard
UNICODE is a standard specification to map all possible characters to a numeric value, in order to cover all possible languages in a unique character set. UNICODE defines the mapping of characters to integer codes, but it does not define the exact implementation (that is the encoding) for a character.
Several character sets are based on the UNICODE standard, such as UTF-7, UTF-8, UTF-16, UTF-32, UCS-2, and UCS-4. Each of these character sets use a different encoding method. For example, with UTF-8, the letter Æ is encoded with two bytes as 0xC3 and 0xB6, while the same character will be encoded 0x00C6 with UTF-16.
When Microsoft™ Windows® users talk about UNICODE, they typically mean UCS-2 or UTF-16, while UNIX™ users typically mean UTF-8.