Many different monsters lurk in Sanctuary, and they will try to kill you in diverse ways. Some will spit acid or beat hurricanes at you from afar; others will lunge, hoping to knock you back and rip you apart before you can react.
Unique monsters have their names displayed in purple. They're usually more powerful than champions: they have more health, do more damage, and often have multiple abilities.
Rare monsters have their names displayed in yellow. Rare monsters have more life than normal for their type, possess a special affix, and are usually surrounded by a pack of three to four minions who are weaker but possess similar abilities.
In addition to the massive variety of normal monsters roaming the land, you'll regularly come face-to-face with champion monsters. Champion monsters have their names displayed in blue. They travel in packs and are more powerful than regular monsters: they take more hits to kill, do more damage, and have abilities called affixes, which enable them to do things like teleport away from your attacks or unleash bolts of lightning when they're struck.