ASCII code vs Unicode

ASCII Code

ASCII defines 128 characters with 7-bits, because the center of the computer industry was in the USA at that time and 7 bits was originally sufficient for the binary representation of English language.

ASCII Extended

Some people extend the 7-bit ASCII code to 8-bit in order to encode more characters to support their language, such as French.
The name for this are often referred to as "extended ASCII" or "8-bit ASCII".

Unicode on the rise

ASCII Extended solves the problem for Latin alphabetical based languages but what about the others that need a completely different set of alphabets, such as Greek, Russian, Arabic, Chinese or Japanese?

Unicode is a superset of ASCII.
Unicode defines up to 221 characters

UTF Varieties Explained

Unicode encoding: UTF-8 vs UTF-16 vs UTF-32

UTF-8 and UTF-16 are variable length encodings.

In UTF-8, a character may occupy a minimum of 8 bits.
In UTF-16, a character length starts with 16 bits.
UTF-32 is a fixed length encoding of 32 bits.

UTF-8 uses the ASCII set for the first 128 characters. That's handy because it means ASCII text is also valid in UTF-8.

Mnemonics:
UTF-8: minimum 8 bits.
UTF-16: minimum 16 bits.
UTF-32: minimum and maximum 32 bits.

Java Supports Unicode

Java Char provides support for Unicode with 2 bytes size, ranging from 0 to 65535.

More...

ASCII and Unicode are two character encoding standards on how to represent characters in binary code.
The main difference between the two is how they encode the character and the number of bits that they use for each. 

ASCII originally used seven bits to encode each character. This was later increased to eight with Extended ASCII to address the apparent inadequacy of the original to encode languages other than English. 

In contrast, Unicode can choose between 32, 16, and 8-bit encodings. Using more bits lets you use more characters at the expense of larger files while fewer bits give you a limited choice but you save a lot of space. Using fewer bits (i.e. UTF-8 or ASCII) would probably be best if you are encoding a large document in English.

Unicode solves the main problem arose from the many non-standard extended ASCII programs. Unless you are using the prevalent page, which is used by Microsoft and most other software companies, then you are likely to encounter problems with your characters appearing as boxes. Unicode eliminates this problem as all the character code points were standardized.

Another major advantage of Unicode is that at its maximum it can accommodate a huge number of characters. Because of this, Unicode currently contains most written languages and still has room for even more. This includes typical left-to-right scripts like English and even right-to-left scripts like Arabic. Chinese, Japanese, and the many other variants are also represented within Unicode.

In order to maintain compatibility with the older ASCII, which was already in widespread use at the time, Unicode was designed in such a way that the first eight bits matched that of the most popular ASCII page. If you open an ASCII encoded file with Unicode, you still get the correct characters encoded in the file. This facilitated the adoption of Unicode as it lessened the impact of adopting a new encoding standard for those who were already using ASCII.

Summary:
1.ASCII uses an 8-bit encoding while Unicode uses a variable bit encoding.
2.Unicode is standardized while ASCII isn’t.
3.Unicode represents most written languages in the world while ASCII does not.
4.ASCII has its equivalent within Unicode.

Taken from here