When two or more threads of your program are blocked from 
gaining access to a resource and as a result cannot make 
ant progress, such a situation is called Starvation.

The ultimate form of Starvation is Deadlock. Deadlock 
occurs when two or more threads are waiting on a condition 
that cannot be satisfied. Most often a Deadlock occurs when 
two or more threads are each waiting for the other to do 
something.

Semaphores are used to avoid Deadlock situations.