Why do need Unicode when you have Ascii?

Answer 1

ASCII only has 127 standard character codes and only supports the English alphabet. While you can use the extended ASCII character to provide a set of 256 characters and thus support other languages there's no guarantee that other systems will use the same code page, so the characters will not display correctly across all systems (the characters you see will depend upon which code page is currently in use). Moreover, some languages, particularly Chinese, have thousands of symbols that simply cannot be encoded in ASCII.

UNICODE encoding supports all languages and the first 127 symbols are also the same as ASCII, so all characters appear the same across all systems. UTF8 is the most common UNICODE encoding in use today because it uses one-byte per character for the first 127 characters and is therefore fully compliant with non-extended ASCII. If the most-significant bit is set then the character is represented by 2 or more bytes, the combination of which maps to the UNICODE encoding.

Answer 2

Unicode is much more ample than ASCII. ASCII can represent only 127 different characters; there are several different one-byte extensions that can represent about 256 characters. In Unicode, however, it is possible to define over a million characters - I am not sure if all of them are already defined. This makes it possible to use a single character set to represent not only Latin characters (the ASCII characters, but also letters with special symbols, as those used in Spanish, German, French, etc.; as well as characters from languages that use a completely different writing, such as Chinese, Russia, and even Klingon (the made-up language from "Star Trek")!