DATA and TYPES are called Declarative key words.

PATAMEYTERS statement is for defining single selection 
screen field.

TYPES is used to define data Types. and DATA is used to 
define Data Objects. 
Generally TYPES is used to define structures, DATA is used 
to declare Internal Tables

Types is used to declare data types, that is what will be 
the structure of the data object . 
and data word is used to define the data object as a 
structure of a data type.

Type is the template and the data is the implementation.

Parameter is almost like a data with only one difference 
from data that is the value odf parameter is taken from 
user at run time and it is connected with selection-screen.