Basically you take your existing universe, you take your hero and you stick a beard on him, or if it's a her, you dress her up all badgirl sexy- cause 99% of the time the hero if she's a girl is all Christian Science Monitor and no sex.
Often a subset of Bizarro Universe, it is an Alternate Universe where Good and Evil characterisations are reversed, but is otherwise the same as the "real" universe - except where logically derived from this change in morality. As an example, in Bizarro World, the earth is a cube. In the mirror universe, the earth is a sphere, but The Captain has a cool eyepatch.-TV Tropes
The most often adverted to Mirror Universe is the Star Trek one, of course- complete with bearded Spock. :)
