UCS is short for Universal Character Set.
UCS-2 and UCS-4 are legacy fixed-width character encodings for Unicode.
UCS-2 is a fixed-width 2-byte character encoding. It was the predecessor to the variable width encoding UTF-16.
UCS-4 is a fixed-width 4-byte character encoding.
UTF is the preferred character encoding for Unicode characters. It can be variable width, or fixed width.
UTF-8 is a variable-width: 1,2,3 or 4-byte encoding.
UTF-16 is a variable-width: 2-byte or 4-byte encoding.
UTF-32 is a fixed-width: 4-byte encoding.