ASCII is a standard code, consisting of 128 7-bit combinations, for characters stored in a computer or to be transmitted between computers.
It stands for A(merican) S(tandard) C(ode for) I(nformation) I(nterchange)
UNICODE is an international encoding standard for use with different languages and scripts, by which each letter, digit, or symbol is assigned a unique numeric value that applies across different platforms and programs.
Differences between ASCII and UNICODE is that
ASCII defines 127 (maybe 128) characters, which map to the numbers 0–126. Unicode defines (less than) 221 characters, which, similarly, map to numbers 0–221 (though not all numbers are currently assigned, and some are reserved).
Unicode is a superset of ASCII, and the numbers 0–126 have the same meaning in ASCII as they have in Unicode. For example, the number 65 means “Latin capital ‘A'”.
Because Unicode characters don’t generally fit into one 8-bit byte, there are numerous ways of storing Unicode characters in byte sequences, such as UTF-32 and UTF-8.