Value type vs Reference type

Reference types are stored on the run-time heap;
they may only be accessed through a reference to that storage. This allows the garbage collector to track outstanding references to a particular instance and free the instance when no references remain.
A variable of reference type always contains a reference to a value of that type or a null reference.
A null reference refers to nothing;
it is invalid to do anything with a null reference except assign it.
Assignment to a variable of a reference type creates a copy of the reference, not a copy of the value being referenced.

Value types are stored directly on the stack, either within an array or within another type.
When the location containing a value type instance is destroyed, the value type instance is also destroyed.
Value types are always accessed directly;
it is not possible to create a reference to a value type.
Prohibiting such a reference makes it impossible to refer to a value class instance that has been destroyed.
A variable of a value type always contains a value of that type. Unlike reference types, the value of a value type cannot be a null reference, nor can it reference an object of a more derived type.
Assignment to a variable of a value type creates a copy of the value being assigned.