This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
In simple terms, if there is a reference like "ArrayList<Animal> anim", you can add a dog, cat, cow, or any other animal into that object, using the add(-) method. You may have a look at the Sun Generics tutorial.