What is AbstractQueuedSynchronizer and which classes are built on it?

AQS gives you one volatile int state, CAS operations on it, and a FIFO queue of parked threads. A synchroniser subclasses it and defines what the state means and when acquisition succeeds. Everything else, the queueing, parking, unparking and cancellation, is inherited.

ReentrantLock uses state as a hold count. Semaphore uses it as a permit count. CountDownLatch uses it as the remaining count and never lets it go back up. ReentrantReadWriteLock packs readers and writers into two halves of the same int. ThreadPoolExecutor.Worker uses it too, as a small non-reentrant lock. FutureTask did until Java 7, and has used its own state field and stack of waiters since.

Note that synchronized is not on the list: monitors are implemented in the JVM, not in Java, which is why a thread waiting on a ReentrantLock appears as WAITING at LockSupport.park rather than as BLOCKED.