What is an Arduino?
b_e_n 
The Extended Family
While your Arduino board sure is pretty, it can't do a whole lot on its own -- you've got to hook it up to something. There are lots of tutorials here on learn as well as the links back in the 'What does it do' section, but rarely do we talk about the general kinds of things you can easily hook into. In this section we'll introduce basic sensors as well as Arduino shields, two of the most handy tools to use in bringing your projects to life.
Sensors
With some simple code, the Arduino can control and interact with a wide variety of sensors - things that can measure light, temperature, degree of flex, pressure, proximity, acceleration, carbon monoxide, radioactivity, humidity, barometric pressure, you name it, you can sense it!
Shields
Additionally, there are these things called shields -- basically they are pre-built circuit boards that fit on top of your Arduino and provide additional capabilities -- controlling motors, connecting to the internet, providing cellular or other wireless communication, controlling an LCD screen, and much more.
For more on shields, check out:
- ShieldList.org
- ShieldStravaganza!!! (A series of videos briefly explaining all of the shields we have at SparkFun) Part 1, Part 2, and Part 3