For Private Bytes and Working Set, read this answer.

Heap size is the size of Managed Heap. If your code constructs new instance of class, it will be allocated on the Managed Heap. for better understanding, search about Stack and Heap Memory Model, which is the fundamental basis of modern computers.

Difference between Private Bytes and Heap Size doesn't represent anything. If your program loads a lot of static/dynamic libraries, the memory needed to load them would not be in Heap and thus Heap Size would not grow. If your program loads lots of data and constructs truckloads of class instances to store it on memory, they will be in Heap and thus Heap Size would grow.