What is the Arduino Shield? A lot of people are saying that the Arduino board is like a mini cpu, if this is the case then the Arduino shields are like the printer, scanner, monitor, mouse and etc to the Arduino boards. Arduino shields are small pieces of PCB boards that expanding the Arduino main board capabilities. Example: An Arduino GPS shield that able to read the GPS coordinate and pass to the Arduino board for action or an Arduino ethernet shield that turn on the network capabilities for your Arduino projects.
Since Arduino is an open-source project, there are a lot of third parties Arduino shields in the market. Here are some of the common Arduino shields that you can get from the market, some of them are really cheap:
LCD Arduino Shield from Amazon for USD13.99
Ethernet Arduino Shield from Amazon at USD39.95
GPS Arduino Shield from Amazon at USD36.66
Arduino shields are built so that they can easily stack over the Arduino main board to reduce implementation space and wiring works.