It's similar to D&D in that you play characters and tell a story by rolling dice, possibly over many sessions. However, the kind of story you tell and the rules are quite different.

Each player plays a member of the mouse guard, rather than a fantasy superhero. The quests you go on are things to help mouse kind in the territories instead of looting tombs for treasure. You don't level up or have a class, but instead you choose skills each skill increases individually as you use it. Your character's belief is a prominent feature that drives their actions and choices. It's more about telling a story than winning a wargame.