An affix is something stuck onto a word, right?
Well, not like this:
No, an affix is when you add extra letters to change the meaning of a word a bit.
Affixes are things like kindness, uncomfortable, gives, children.
So that's an affix...
...except when it's not.
Sometimes these stuck-on bits are clitics.
A clitic is when the stuck-on bit is part of a separate word.
English examples might be the 's in it's (which, as we all know, is short for it is) or the 't in 'twas (it was) or the n't in haven't (have not) or the l' in the French l'histoire (the story).
You may be asking why this matters.
Well, having read rather a lot about clitics, and found out that clitics are all sorts of other things, too, like, for instance, a word which can't be stressed (such as the French word te) I know I'm asking why it matters.
Ah well. It's a lovely clicky word, in any case.
Thing To Use Today: a clitic. Whatever they are, they're all over the place in all sorts of languages. The word is to do with the Greek proklinein, which means to lean forwards.