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 thought of as counters that signal when a resource is available or when it is in use. There are two main types of semaphores: binary semaphores, which can only take values of 0 or 1, and counting semaphores, which can take a range of values. These tools are essential in operating systems and concurrent programming to maintain order and prevent race conditions.