Answer:
UTF-8 and ASCII both are the character encoding.
In a system,every character has some binary representation,these are the method to encode them.Earlier only ASCII was there, for every character it uses 8 bits to represent.In ASCII only 8 bytes were there i.e 2^8 that is 256.We can't represent number beyond than 127 so it generate a need for other encoding to get into,these drawbacks lead to Unicode,UTF-8.
As ASCII codes only uses a single byte,UTF-8 uses upto 6 bytes to represent the characters.So we can save characters which are as long as 2^48 characters. We can read this encoding easily by the help of shift operators and it is also independent of byte order.
As messages on internet were transferred over 7 bit ASCII messages,so many mail servers removed this encoding.