<p>What are the basic differences between a semaphore &amp; spin-lock?</p>

<p>When would we use a semaphore over a spin-lock?</p>

very simply, a semaphore is a "yielding" synchronisation object, a spinlock is a 'busywait' one. (there's a little more to semaphores in that they synchronise several threads, unlike a mutex or guard or monitor or critical section that protects a code region from a single thread)
You'd use a semaphore in more circumstances, but use a spinlock where you are going to lock for a very short time - there is a cost to locking especially if you lock a lot. In such cases it can be more efficient to spinlock for a little while waiting for the protected resource to become unlocked. Obviously there is a performance hit if you spin for too long.
typically if you spin for longer than a thread quantum, then you should use a semaphore.