What forms a contraction?

A contraction is a shortened form of two Words, or of a combined Word, where one or more letters is replaced by an apostrophe. Typically contractions are used on verbs combined with other verbs, adverbs, or pronouns.
Examples:
don't = do not
he's = he is
can't = cannot
would've = would have (auxiliary verbs)
won't = will not (from old form wonnot)