The main difference between wait() and sleep() is that wait is an Object level method, whereas sleep() is a static method in Thread class. A waiting thread can be woken up by another thread by calling notify() on the monitor which is being waited on. But a sleeping thread cannot be woken up.

A wait() and notify() has to happen within the same block that is synchronized on the monitor object.

When we call wait() the current thread releases the monitor and goes to waiting state. Then another thread calls notify() to wake it up.

In case of sleep() current thread does not release the monitor or locks. It just sleeps for some pre-defined time period.