This week's book giveaway is in the Testing forum. We're giving away four copies of The Way of the Web Tester: A Beginner's Guide to Automating Tests and have Jonathan Rasmusson on-line! See this thread for details.
It is something called Coding to the interface.
With the first- We could assign instances of anything which implements List- ArrayList, LinkedList, Vector to the list reference
In the second case- We can assign instances of ArrayList only.