|Unicode is a vast specification covering many things. Encluding serveral representations of some very broad (many natural languages) character sets.
There are also associated encodings for those character sets.
Some encodings have a fixed size and some are variable.
A variable size encoding has some byte sequences which represent specific characters and other byte sequences which are used as flags to indicate that additional bytes are needed to determine the actual character.
A multi-byte character set might either mean a fixed size representation of a character set but normally means a variable sized encoding which started with a single byte for the initial encoding. UTF8 is a variable sized encoding and can thus also be considered a multibyte character set.
For coverage of this specific point see the following