Shields are boards that can be plugged on top of the Arduino board extending its capabilities.

There are dozens of shields out there, all of which make your Arduino more than just a development board with a blinky LED.

Every Arduino shield must have the same form-factor as the standard Arduino. Power and ground pins on one eight (previously six) pin header, and analog pins on a six-pin header next to that. Digital pins cover the other edge on the other side, an eight-pin header separated from a 10-pin by that weird 0.5" spacing. Some shields also require a connection to the Arduino’s ICSP header (the 2x3 programming header on the end).

Some shields use every pin on the Arduino, while others only use a couple. When stacking shields, it’s important to make sure they don’t use overlapping pins.