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What is basic difference between semaphore and monitor in operating system?

Answer

The wait and signal operations on condition variables in a monitor are similar to P and V operations on counting semaphores. A wait statement can block a process's execution, while a signal statement can cause another process to be unblocked. However, there are some differences between them. When a process executes a P operation, it does not necessarily block that process because the counting semaphore may be greater than zero. In contrast, when a wait statement is executed, it always blocks the process. When a task executes a V operation on a semaphore, it either unblocks a task waiting on that semaphore or increments the semaphore counter if there is no task to unlock. On the other hand, if a process executes asignal statement when there is no other process to unblock, there is no effect on the condition variable.
Another difference between semaphores and monitors is that users awaken by a V operation can resume execution without delay. Contrarily, users awaken by a signal operation are restarted only when the monitor is unlocked.
In addition, a monitor solution is more structured than the one with semaphores because the data and procedures are encapsulated in a single module and that the mutual exclusion is provided automatically by the implementation.
(excerpted from http://ei.cs.vt.edu/~cs5204/sp99/monitor.html)