How does synchronized compare with explicit locks?

A synchronized method or block acquires the object's monitor. The JVM releases it automatically when the block exits, including on exception. ReentrantLock is a class in java.util.concurrent.locks that requires an explicit unlock in a finally block.

private final ReentrantLock lock = new ReentrantLock();

public void transfer(Account from, Account to, long amount) {
    lock.lock();
    try {
        from.withdraw(amount);
        to.deposit(amount);
    } finally {
        lock.unlock();
    }
}

What ReentrantLock adds

• tryLock with an optional timeout instead of blocking forever
• lockInterruptibly so a thread can respond to cancellation
• An optional fairness policy, which reduces starvation but lowers throughput
• Multiple Condition objects for separate wait sets
• ReentrantReadWriteLock for reader-writer separation

Prefer synchronized for simple mutual exclusion because it is harder to misuse. Reach for ReentrantLock when you need non-blocking acquisition, timeouts, interruptibility or multiple conditions. Both are reentrant: the same thread can acquire the lock again.