What are EDT'S and the purpose of it?

EDTs are user defined types created on the basis of Primitive types such as real, string, integer, date and composite type like container. We can create a new type called name and its type as string and this type name again can be used any where the primitive and composite types used.
Our code would be easier to read, since variable have meaningful data type and also properties of the new type would be used as the default in forms and reports. ALL STANDARD CONVERSIONS ARE APPLICABL