Aurdino Compatible 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.
Want to connect your Arduino to the Internet and post to Twitter? There’s a shield for that. Want to make your Arduino an autonomous rover? There are shields for that. There are dozens (hundreds?) of shields out there, all of which make your Arduino more than just a development board with a blink LED.
Many Arduino shields are stackable. You can connect many shields together to create a “Big Mac” of Arduino modules.