How does fair-use law work?

This is an excel­lent write-​​up about how fair use works, along with its com­plex­i­ties (and areas where it is more straight­for­ward, gen­er­ally where courts have already ruled on a very sim­i­lar use previously):

If you kill some­one, you’ve com­mit­ted mur­der, right? Yes — unless he was about to shoot you first, in which case we call it self-​​defense. Fair use takes that same con­cept to copy­right law. It is all about jus­ti­fi­ca­tion, and this is a key to under­stand­ing it. Fair use allows use of a work that would ordi­nar­ily con­sti­tute infringe­ment, if that use is jus­ti­fied (or excused, if you like) with some com­pelling reason.

How does fair-​​use law work? — By Tim Wu — Slate Magazine.

Recommended read­ing.