The existing answers already explain a lot of details, but here's a very short answer with the most direct explanation and example.
Unicode is the standard that maps characters to codepoints.
Each character has a unique codepoint (identification number), which is a number like 9731.
UTF-8 is an theencoding of the codepoints.
In order to store all characters on disk (in a file), UTF-8 splits characters into up to 4 octets (8-bit sequences) - bytes.UTF-8 is one of several encodings (methods of representing data). For example, in Unicode, the (decimal) codepoint 9731 represents a snowman (☃), which consists of 3 bytes in UTF-8: E2 98 83
Here's a sorted list with some random examples.