A character encoding specifies how characters are represented by numerical units and ultimately stored or transmitted as bytes. Unicode assigns code points; UTF-8 is one character encoding for those code points. Keeping identity and representation separate lets the same text have several byte representations.[ref]
UTF-8, a transformation format of ISO 10646 by Francois Yergeau. §3, UTF-8 definition and encoding table
For example,
U+00E9 becomes hexadecimal C3 A9 in UTF-8. A reader using an incompatible character encoding can interpret those two bytes as two unrelated characters. Correct decoding therefore requires the character encoding, and invalid sequences require an explicit error policy. The character encoding alone does not determine the font or glyph used to display the result.