Unicode is a character set, and the only character set really worth speaking of. The Unicode character set includes almost every character in every writing system on Earth. A string is a piece of text, i.e. an ordered sequence of characters all taken from the same character set.

A character encoding is a mapping/function/algorithm/set of rules which can be used to convert a string into a sequence of bytes and back again.

A character set may have multiple encodings. UTF-8 and UTF-16 are two possible encodings of the Unicode character set.