"Pot" magnets, sometimes known as "cup" magnets, are enclosed in a steel "cup", or "pot". This means that the poles are positioned on a single face, with one pole in the form of a ring surrounding the other pole.
This configuration results in the following:
The back of the magnet has almost no magnetic field associated with it, making the magnet useful where stray fields are not welcome.
The field is confined largely to the gap between a centre pole and the ring around it. Again, this makes the magnet useful where stray fields are not welcome.
Unlike standard magnets, the back of a pot magnet can be sunk into a cavity in steel without losing its pull at the surface of the steel.
The magnet grips very well when placed right against flat steel, but at even a small distance away, it has quite reduced pull. To clarify, they are the worst type of magnet for pulling from a distance.
It performs better than a similar size standard magnet when pulling to thin sheet steel.
Their most common application is for cupboard doors, pulling towards a steel washer, bracket, or plate.
They repel strongly from each other when close, but are quite unsuitable for attracting towards each other.
The "cup" protects the quite brittle magnet material against fracturing during impact.
An application for larger ones is for mounting a heavy item against a shipping container, commercial vehicle, heavy machinery, or other solid steel construction.
For their pull they are often a cheaper option than standard magnets.