This set has become the new D/P standard for Skarmory. Roost gives Skarmory improved tanking ability compared to Advance, when its only recovery methods were Rest and Leftovers.
Brave Bird or Drill Peck are there to let Skarmory deal damage when called for. Brave Bird is the preferred attack as it packs more of a punch, and that helps running off of Skarmory's mediocre Attack. Drill Peck can be used if you can't afford to take the recoil, however.
Roost lets Skarmory recover HP. In situations where Skarmory is faster than its opponent, Skarmory can use Roost to remove its Flying type for the rest of the turn in order to reduce damage from things like Stone Edge and Thunderpunch.
The choice between Spikes and Stealth Rock depends on your team, your preferences, and what bothers your team more. Spikes takes longer to fully set up, but will do more damage to things that aren't weak to Rock. If you have more problems with things like Gyarados and Salamence, then Stealth Rock will probably work better.
Whirlwind allows Skarmory to force things that are using stat raising moves to switch out, and simultaneously makes whatever comes in take damage from Spikes or Stealth Rock. If whatever is forced out is ineffective against Skarmory, you can then either use Whirlwind again or use Brave Bird.
Your choice of item depends on whether you are playing under item clause, and if you're afraid of Magnezone. Leftovers will give Skarmory better tanking abilities, while Shed Shell will allow Skarmory to give Magnezone (or the rare Probopass) the slip.]