This document discusses semaphores, which are integer variables that coordinate access to shared resources. It describes counting semaphores, which allow multiple processes to access a critical section simultaneously up to a set limit, and binary semaphores, which only permit one process at a time. Key differences are that counting semaphores can have any integer value while binary semaphores are limited to 0 or 1, and counting semaphores allow multiple slots while binary semaphores provide strict mutual exclusion. Limitations of semaphores include potential priority inversion issues and deadlocks if not used properly.