Advantages of Iterator over forEach

1. Iterator is a design pattern and is a light-weight container.
2. Iterator doesnt need to know what the collection is (i.e. a List or sth else )
3. Using iterator ,we can remove the current element. The for-each loop hides the iterator, so you cannot call remove. Therefore, the for-each loop is not usable for filtering.
4. Similarly, forEach is not usable for loops where you need to replace elements in a list or array as you traverse it, but Iterator can be used.
5 .Finally, forEach is not usable for loops that must iterate over multiple collections in parallel.

http://java.sun.com/j2se/1.5.0/docs/guide/language/foreach.html

Fail fast and fail safe iterator

Fail Fast :
When we iterate over a collection, if during iteration, we modify the collection , then the iteration halts and we get "ConcurrentModificationException" (e.g Hashtable)

Fail Safe :
During iteration, a separate copy of the colleciton object is created and iteration occurs on that. So if we modify it during the iteration process, it wont throw an exception.(e.g HashMap)

Best Explanation (with example) : http://www.certpal.com/blogs/2009/09/iterators-fail-fast-vs-fail-safe/