Mass Effect 3: The Differences Between Game Modes
Mass Effect 3's developers added several features that weren't found in the first two games in the series. These different modes will affect how the entire game plays out, and what kind of experience you'll have.
Action Mode focuses on the fighting rather than Mass Effect 3's interactive story. Shepard will automatically respond to conversational options without player input.
This is the "traditional" Mass Effect mode. Conversational choices are made manually, allowing you to shape Shepard's character.
Story Mode is identical to RPG Mode, with one change: the combat difficulty has been turned down. This will allow you to focus on the story and not have to sweat the combat.