Champions are a subset of Rare enemies, with proper names and more powerful abilities. They often have their own modifiers, which do not spawn on other enemies. They can be recognized by their names, minimap icon, and an orange highlight.
Once defeated, Champions always drop an item with a sealed Champion Affix based on the Champion's modifiers. Champion Affixes only occur on items, they cannot be obtained as shards via Rune of Shattering or Rune of Removal.