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.

c# provides a set of datatypes. datatypes of c# contain two categories one is value type and another is reference type.

value type is a variable. datatype value store in variables while a variable of a reference type stores a reference to the data.

both value type and reference type have one common thing - both derived from system.object.