Semaphore is a mechanism is an RTOS to serialize access to 
a resource/resources for all the threads in the system.
There are 2 types of semaphore
1. counting semaphore - this is used when a set of 
resources have to be shared between many threads. The 
semaphore is initialized with the number of resources. Each 
time a thread tries to access the semaphore, its value gets 
decremented until it becomes 0. After this any try to 
access the semaphore will block the calling thread. 
2. binary semaphore - this is used to serialize access to a 
single resource i.e the semaphore has only two values 0 and 
1. 
A mutex is similar to a binary semaphore except that it has 
an owner i.e the thread where it was created. A mutex can 
be freed only by this thread. This is not the case for 
binary semaphore which can be freed by any thread.