A female lead character who kicks ass.
A shero is different from a heroine in that a heroine is merely the main female character, to be a shero one must do a certain amount of kicking ass and taking down names. Lucy from a Tale of Two Cities and Barbara from Night of the Living Dead were both heroines. Sheros they most certainly were not.
Lara Croft is the shero of Tomb Raider.
Dude, Aeon Flux is the freakin' awesomest shero of all time!
Jessica Alba epitomized a shero as Max in Dark Angel.