Spatial discretization in JOREK
Start with Introduction to spatial discretization. The page Poloidal discretization gives a much more detailed explanation of the discretization on the poloidal plane, pinpointing also why the particular formulation used has been chosen (i.e. which are the advantages).
There is also an introduction to Bezier curves and surfaces at this page.
Finally, in the Spatial discretization and mesh a brief description of how Bezier finite elements are used when building the poloidal mesh.