Each constant, variable, array, expression, or function reference
  in a Fortran statement represents typed data.  The data type of
  these items can be inherent in their constructions, implied by
  convention, or explicitly declared.

  Each data type has a name, a set of associated values, a way to
  denote the values, and operations to manipulate and interpret these
  values.

  There are two categories of data types:  intrinsic and derived.
  The names of the intrinsic data types are predefined and are always
  accessible.  Derived data types are user-defined data types that
  are made up of intrinsic or derived data types.