Pot core:

Still more inductance can be obtained per turn with a pot core. It is the shell of ferromagnetic material which wraps around the loop shaped coil. The core comes generally in 2 halves. You wind coil inside one of the halves, and then bolt the 2 together. The final core surrounds the loop completely, and the magnetic flux is confined to core material.

Figure--   Exploded view of pot core.

Like toroid, the pot core is self shielding. There is basically no coupling to external components.  A pot core can be used to wind up single, high inductance coil; some times the value can be above 1H.

In a pot core transformer, primary and secondary should always be wound on top of, or right next to, each other; this is can't be avoided due to the geometry of shell. Thus, the interwinding capacitance of the pot core transformer is always high.

Pot cores are useful at lower frequencies. They are usually not employed at the higher frequencies because it is not necessary to obtain that much inductance.