Brushed DC: simple, mechanical commutation
A brushed DC motor generates torque by running current through a rotating coil (the armature) sitting inside a fixed magnetic field, and it needs the current direction in that coil to flip every half rotation to keep producing torque in the same direction. Physical brushes, spring-loaded carbon contacts, do that flipping mechanically as the armature spins past them. This is what makes brushed motors so simple to drive: apply a voltage across two terminals and the motor spins, no electronic commutation logic required. That simplicity is also the source of every brushed motor's weaknesses. The brushes physically wear down through friction and arcing, which limits motor lifespan and creates electrical noise. The armature itself has to carry current and rotate, which gives it more rotating mass (inertia) than a comparable brushless design.
Despite those downsides, brushed motors remain common in cost-sensitive, low-duty-cycle applications, hobby robotics, toys, and simple actuators, because a brushed motor plus a basic H-bridge driver is one of the cheapest ways to get controllable rotation. They're also easy to reason about: torque is roughly proportional to current, and speed is roughly proportional to applied voltage, which makes them forgiving for beginners to model and control even without sophisticated electronics.