What is difference between fail-fast and fail-safe Iterators?

• Fail-fast Iterators throws ConcurrentModificationException when one Thread is iterating over collection object and other thread structurally modify Collection either by adding, removing or modifying objects on underlying collection. 

• They are called fail-fast because they try to immediately throw Exception when they encounter failure. 

• On the other hand fail-safe Iterators works on copy of collection instead of original collection so they dont throw any Exceptions.