I didn't know what java.util.stream was either, but this document made it clear: http://www.oracle.com/technetwork/articles/java/ma14-java-se...
For those who don't know Java and C#: All regular collections now offer a .stream() (and .parallelStream()) method that returns the collection as Java 8 Stream<T> instance. With the help of lambda expressions, objects of a stream can be converted/mapped, filtered, sorted, aggregated, reduced, flatmapped, etc. by chaining methods on the stream. You can also create finite or infinite streams without a collection (Like of numbers ranges, random values or anything else you can think of).
I often end up writing complex stream method chains just to break them up in the end, thinking that they are too difficult to understand for others.
It provides fast distributed computation as an infrastructure component - Jet is fully embeddable in your application.
Disclaimer: I am one of the engineers who worked on this.