Java (High Level Concurrency) tutorial

Just worked through the last part of the online Java Concurrency tutorial. Understood the java.util.concurrent.locks package (i.e. Lock, Reentrant etc classes including trylock(), unlock() etc methods) and java.util.concurrent.atomic package (i.e. AtomicInteger etc classes including incrementAndGet(), decrementAndGet() etc methods). Didn't quite get the stuff about Executors and Thread Pools! Also, couldn't do the Questions at the end of the tutorial so there's a clear sign I need more practice!!