Like intrinsic data types, a Fortran 95/90 derived data type has a
  name, a set of associated values, a way to denote the values, and
  operations to manipulate and interpret these values.

  The names of the intrinsic data types are predefined, while the
  names of derived types are defined in derived-type definitions.

  A derived-type definition specifies the name of the type and the
  types of its components.  A derived type can be resolved into
  "ultimate" components that are either of intrinsic type or are
  pointers.

  The set of values for a specific derived type consists of all
  possible sequences of component values permitted by the definition
  of that derived type.  Structure constructors are used to specify
  values of derived types.

  Nonintrinsic assignment for derived-type entities must be defined
  by a subroutine with an ASSIGNMENT interface.  Any operation on
  derived-type entities must be defined by a function with an
  OPERATOR interface.  Arguments and function values can be of any
  intrinsic or derived type.