Arduino board and ardunio shield

Projects Discussion and Showcase Education and Teaching
1

I am new to arduino.Can someone explain me what is the difference between arduino board and shield.Why are they used???where are they used???

2

http://arduino.cc/en/main/boards

"Shields are boards that can be plugged on top of the Arduino PCB extending its capabilities. The different shields follow the same philosophy as the original toolkit: they are easy to mount, and cheap to produce."