What is the difference between a hub motor and mid-drive motor?

A hub motor sits in the wheel hub (front or rear) and pushes or pulls the bike directly. It's simpler, cheaper to maintain, and harder to damage. A mid-drive motor sits at the bottom bracket and drives the chain, using the bike's gears for mechanical advantage. Mid-drive motors are more efficient on hills and trails but cost more and put more stress on the drivetrain.