In the object world, Inheritance is natural thing. However the flat data model cannot be easily mapped to the hierarchy object model. Entity Framework provide TPT and TPH to solve this problem.
Below is a diagram I draw based on the MSDN Table-per-type sample. The right side is a list of entity types in the source layer while the left side is a list of entity types in the entity layer ( or conceptual layer). In the conceptual layer, the DeptMusic, DeptEnginerring and DeptBussiness are derived from base type Department. That is, four tables have corresponding entity types. Therefore this inheritance solution is called Table-per-type ( TPT ). Although the Department and DeptBusiness tables have no relationship with each other, the primary keys of these tables are mapped to the same DepartmentID of the Department base type.
Table-per-hierarchy ( TPH ) is another way to create inheritance entity model in the EDM. The MSDN sample uses one person to hold the persisted data from the Student, Administrator, instructor and Person entities. As you might notice, Person is the base type in the hierarchy while Student, Administrator and Instructor are derived types. The PersonCategory column of the Person table is called discriminator column. If the value of PersonCategory is 0 in the database, when the data is materialized to object via Entity Framework, a person object will be created. A value of 1,2,3 indicate the Student, Instructor and Administrator types respectively. You can use Entity Mapping Detail window to specify discriminator columns and values.
source http://lwang00.wordpress.com/2007/11/04/entity-mapping-table-per-type-and-table-per-hierarchy/
