DOMAIN -> Region over which a PDE is to solved as defined by the problem. Contains the limits of the problems and the mesh or grid on which it will be solved. 

FUNCTIONSPACE -> Defines where on a DOMAIN a function will be evaluated. Specifically what part of the mesh. A number of FUNCTIONSPACES exist and these are defined in the user guide.

DATAOBJECTS -> when a function is applied to DOMAIN and a FUNCTIONSPACE a DATA OBJECT is returned. It contains the values of the function when evaluated at the points defined by the union of a domain and a function space.


