Do have a look at the Arduino website
The Arduino platform is huge and keeps expanding. There is a wide array of micro-controller boards, shields, add on boards and sensors available. It is a cheap option for learning how to program micro-controllers and start building robotics projects.
The compiler for programming
Sensors allow your robot (or project) to find out more about the environment surrounding it. It can then make decisions about what to do, or report what it reads – For instance read the temperature and humidity and print the value on an LCD screen.
Shields fit on top of an Arduino board and give added functionality. Some shields even allow one to fit another shield on top.