A Unicode string is a sequence of codepoints (or characters).

A codepoint (or character) is a numeric id that can be represented by many ways (UTF-32 : always 4 bytes ; UTF-8 : only one byte for codepoints < 128 ; from 2 to 4 bytes for codepoints >= 128 ; ...).

A glyph is what you display on the screen : a Unicode string is displayed as a sequence of glyphs. A glyph can be a single character, or the combination of 2 or more characters.

e.g., 10 glyphs on the screen can be represented as 11 characters (the two last ones being composed in a single glyph). Depending on the underlying "physical" encoding, these 11 characters can occupy 44 bytes (UTF-32) (with a O(1) access to substrings) or, say, 25 bytes (UTF-8) (with a O(n) access to substrings), or ...

In a few words : characters have a meaning in Unicode, but they don't match well with bytes (and even with physical representation) and, sometimes, with the way things are representing on the string.