Difference between ‘combining characters’ and ‘grapheme extenders’?

[Mathias Bynens]

What is the difference between ‘combining characters’ (http://www.unicode.org/faq/char_combmark.html) and ‘grapheme extenders’ (http://www.unicode.org/reports/tr44/#Grapheme_Extend) in Unicode?

They seem to do the same thing, as far as I can tell – although the set of grapheme extenders is larger than the set of combining characters. I’m clearly missing something here. Why the distinction?

I’ve also posted this question on Stack Overflow: http://stackoverflow.com/q/21722729/96656