The µC or Mircrocontroller is a powerhouse of multiple small processors in one package. The art of harnessing the power of the chip is the gateway to mastering embedded systems.
As I see it embedded system need to interact with the outside world in many different ways. Communicating with the real world is very necessary.
The secret of good utilization of the µC power is using “Interrupts”. For example, a µC can be receiving serial data, displaying on a 7-segment display, generating PWM, executing an analog to digital conversion and executing main code all at the same time! We need to remember all the components mentioned are actually functioning at the same time because they are all different entities packaged in the same body. Getting them to interact with the main routine is through interrupt service routines, that means end of reception of serial data, end of ADC conversion or a display update.
The difference in the writing a simple main program and an interrupt driven program is like writing a program on a sheet of program and writing many programs on multiple sheets of paper and making a book. Now you can jump from page to page and execute code putting a bookmark here and there. Like jumping to the index of the book, etc.