What are condition variables and how are they used with mutexes?

Condition variables allow threads to wait for specific conditions while holding a mutex. Operations: wait() atomically releases mutex and sleeps until signaled, reacquires mutex before returning; signal() wakes one waiting thread; broadcast() wakes all. Always use with mutex and check condition in while loop (spurious wakeups). Example: producer signals 'not empty' when adding, consumer waits on 'not empty' when buffer empty. Enables efficient waiting without polling.