A Detachment is a set of rules with benefits. Such as a Combined Arms Detachment. A Formation is a sub-unit, which doesn't really affect the primary list at all... they're just along for the ride.
The Decurion Detachment skips using a Combined Arms or whatever, and its benefit is that it doesn't give two craps about the standard FOC. It uses these special 'Auxillary' units instead. You don't take 0-3 Elites, you take 3 Deathbringer Flights, 2 Flayed Ones, and a Judicator Battalion. For example. I also heard a rumor that you give up Objective Secured for Rerolling 1s on the protocols.