Darth Maul = Dark Weapon (a maul is a type of weapon, or can be used describe an act of attacking someone; essentially he's the "weapon of Darth Sidious")
Darth Tyranus = Dark Tyrant (which is pretty generic for a Sith, though in this case it refers to Count Dooku's political ambition, which he persues now that he no longer goes by the Jedi Code)
Darth Vader = Dark Father (which makes sense when you look at episodes 4-6, though Ep. 3 may have other meanings behind it)
Darth Sidious = Dark Insider (he's taking over the Republic from within, exploiting the corruption to elevate his "Palpatine" public persona)
Darth Revan/Malak = Dark Revan/Malak (they simply prefixed "Darth" to their names like a title, meaning they are now dark siders)
Darth Sion = ??? (not sure)
Darth Nihilus = Dark Nihilist (an extreme nihilist, consumed by hunger and barely bound to even the Sith ways, he'd slowly erase all life in the galaxy if left unchecked)
Darth Traya = Dark Betrayer (she is one who was betrayed and now lives through betraying others)