A Magnetic Coupling is a coupling that transfers torque from one shaft but using a magnetic field rather than a physical mechanical connection. Magnetic shaft couplings preclude the use of shaft seals, which eventually wear out and fail from the sliding of two surfaces against each other. Magnetic couplings are also used for ease of maintenance on systems that typically require precision alignment, when physical shaft couplings are used, since they allow a greater off axis error between the motor and driven shaft.
Magnetic Couplings are most often used for liquid pumps and propeller systems, since a static, physical barrier can be placed between the two shafts to separate the fluid from the motor operating in air.
Some aquarium pumps are Magnetic Drive Pumps. They use a magnetic coupling between the motor on the dry side of an aquarium wall and the propeller/impeller in the water on the other side of that aquarium wall. With two face-to-face magnetized disks, one driving magnet on the dry side, and another driven magnet on the wet side of the glass. There are two options for designing the magnetic pattern on each disk. One option balances the (attractive) section that transfers torque, with magnetic repulsion near the axis, to nearly cancel out axial load.
The other option designs the magnetic pattern to maximize the torque and uses a mechanical thrust bearing to resist the attraction between the magnetized disks.