critical section anything...file,code,memory,register,colddrink,food which is "shared" by two processes..
mutual exclusion...if a process is inside the critical section.....other process shouldnt enter inside...like u r in a washroom..till that time no one should enter..
semaphore...a "variable"...a protected variable on which read modify etc happens automatically in kernal mode....like before entering washroom u have kept a bodygaurd ouside who will prevent others from entering..and when u comes out...it is removed...P and V are the operations to do so...