The process of transitively tracing through all pointers to
actively used objects in order to locate all objects that
can be referenced, and then arranging to reuse any heap
memory that was not found during this trace. The common
language runtime garbage collector also compacts the memory
that is in use to reduce the working space needed for the heap.

Garbage Collector is a process of releasing the memory used 
by the objects, which are no longer referenced. 
Programmatically Invoking the Garbage Collector by using 
GC.Collect () method which is from System.GC class.