join() method will wait for the current thread to complete execution.

Let us assume that t1 and t2 are two executing threads. If t1 calls join() on t2 i.e.; t2.join() immediately t1 will enter into waiting state until t2 completes its execution.

yield() method gives a hint to the scheduler that the current thread is willing to yield its current use of a processor.

yield() method pauses the currently executing thread temporarily for giving a chance to the remaining waiting threads of the same priority to execute. If there is no waiting thread or all the waiting threads have a lower priority then the same thread will continue its execution. The chance for execution of the yielded thread is decided by the thread scheduler whose behavior is vendor dependent.