Their appearance varies wildly; they do not necessarily or usually resemble actual keys.
They were originally conceived as free passes into (and, crucially, out of) hell, like intelligent identity cards, but they can be used to move between other worlds.
They have some practical drawbacks, in that they cannot be used to travel anywhere - it has to be somewhere the key has been before, and left a spatial marker, called a gate. A key cannot open another key's gates, but some gates (including the entrances to hell) are 'public' and can be used by any key.
Known keys belong to, or are held by: