Unicode is a global standard for character encoding and is the most commonly used character set today.
Basic ASCII Character Set
The basic ASCII character set uses 7-bits for each character
27 = 128
Extended ASCII Character Set
The Extended ASCII character set uses 8-bits for an additional 128 characters
28 = 256
Unicode is the new standard for representing characters of all the languages of the World.
ASCII character encoding is a subset of Unicode.
The Unicode standard defines UTF-8, UTF-16 and UTF-32
UTF-8 represents 256 distinct characters (popular encoding used on the web).
UTF-16 represents 65,536 distinct characters (used by Java and Windows).
UTF-32 represents 4,294,967,296 possible characters, enough for all known languages (UTF-8 and UTF-32 are used by Linux and various Unix systems).
Unicode advantages over ASCII
More languages or all(modern) languages can be represented in one character set.
Improved portability of documents in Unicode as each character has an unique representation in Unicode.