Arduino is an open-source electronics platform based on easy-to-use hardware and software. Arduino boards are able to read input: for example light on a sensor, a finger on a button, or a Twitter message; and then converting it into an output: activating a motor, turning on an LED, publishing something online. You can tell your board what to do by sending a series of instructions to the microcontroller on the board. Arduinos are often super modular, which makes them very "Plug and Play". This allows you to create handy machines relatively quickly. Arduinos run the Arduino programming language (based on Wiring), which is written in the Arduino Software IDE (based on Processing).
What does a Arduino developer do?