Retribution Paladins are an order or type of paladin. Most paladins are a terrific aid in the war — skilled in combat, riding, and even doing a bit of healing here and there — according to Brann Bronzebeard, "Except those damn "Retribution Paladins" — most of 'em can't heal worth a damn. Bah.". Retribution Paladins are focused on damage output, two-handed weapons, holy damage, and damage avoidance rather than healing.
Judging by the Retribution talent tree, there are Retribution Paladins within Knights of the Silver Hand and the Blood Knight order. However, considering that the term "Retribution Paladin" is capitalized, it is likely its own order or suborder.