A semaphore is a synchronization tool used in computer science to manage access to shared resources by multiple processes. It helps prevent conflicts and ensures that only a certain number of processes can access a resource at the same time. Semaphores can be binary, allowing only one process at a time, or counting, which permits a specified number of processes. In programming, semaphores are often implemented using mutexes or condition variables. They are crucial in operating systems and concurrent programming, helping to avoid issues like deadlocks and race conditions by controlling how processes interact with shared data.