If you are confused about the Divisional Round, have a look at this: http://www.nfl.com/playoffs/playoff-picture
The 6 remaining teams in each conference are ranked with a "seed" (#1 being the best, #6 being the worst). For the Divisional Round, the #1 seed plays the worst surviving seed from the Wild Card Round, and the #2 seed plays the best surviving seed.
So for the AFC, the Patriots can't play the Steelers in the Divisional Round, because the Steelers are #3, meaning there must be a team with a lower seed still in the playoffs. Likewise in the NFC, the Cowboys can't play the Seahawks in the Divisional Round.
The 6 remaining teams in each conference are ranked with a "seed" (#1 being the best, #6 being the worst). For the Divisional Round, the #1 seed plays the worst surviving seed from the Wild Card Round, and the #2 seed plays the best surviving seed.
So for the AFC, the Patriots can't play the Steelers in the Divisional Round, because the Steelers are #3, meaning there must be a team with a lower seed still in the playoffs. Likewise in the NFC, the Cowboys can't play the Seahawks in the Divisional Round.