Arduino is a family of single-board microcontrollers, intended to make it easier to build interactive objects or environments. What the Arduino platform has done is to take what was once a fragmented and expensive market for robotics and microprocessors and become the major platform, largely by virtue of much lower cost and ease of use, leading to higher volume and popularity, and community support behind it. Arduino has made it simple to program their boards with any computer via USB and simple to integrate with a wide array of sensors and devices. The Arduino is great for hobbyists, prototypers, and people just starting out in robotics because of its low cost, ease of use, and large following online. It's easy to learn and teach people to be able to do basic things with the Arduino, yet it's capable enough to do fairly sophisticated things if you as a developer have the capability to take advantage of it.