I think Garbage Collector is a normal priority thread 
running in the application under normal conditions, when 
heap memory is full it takes high priority and traces  
referrance tree and mark the objects which have referrance 
count is zero.usually when object is out of scope it memory 
in heap automatically reclaimed by Garbage Collector(if it 
has no referrances)

Garbage Collector identifies a unused object using "referce 
counting" reference countiing: how many times the object is 
used if the count is zero..its is unuesd....now the gc will 
release the resources..