What's the difference between a bullet journal and an undated planner?

A bullet journal is essentially a blank dot-grid notebook you structure yourself using indexes, future logs, and custom spreads (the Leuchtturm1917 Bullet Journal Edition 2 is built around this system). An undated planner already has weekly or daily layouts printed, but no dates, so you fill them in and can start or pause whenever you want. Bujo gives you maximum flexibility; undated planners give you structure without calendar lock-in.