Undifferentiated Memory

The heap is memory that is dynamically allocated during runtime. Reference types like classes use the heap to store information that is fast to change. The stack is memory that is allocated to be accessed quickly but is less dynamic to use than memory on the heap. Value types like structs use this memory. Undifferentiated memory (going from a few years ago in one of my CS classes) is memory that has not been allocated for either purpose, and is basically a fancy term for "unallocated memory".

int myInt = 25;                 // Allocate on the stack
string myString = "Hello";      // Allocate on the heap

Nullable<int> myNullInt = null; // Creates a null integer on the stack
int? myNullInt = null;          // Shorthand to create a null integer on the stack
string myNullString = null;     // Creates a null string on the heap