Arduino is an open-source hardware prototyping platform. It is widely used today in electronics projects because it is easy to learn, simple in design, well documented and cheaper. We call it platform because it is both hardware circuit as well as piece of software, the IDE. It also has its own programming language. All these open-source components collectively makes the Arduino platform.
Bit of History: It all started at IDII, Ivrea, Italy in 2003 when some students at the institute started working on creating a low cost prototyping tool for digital projects. Arduino’s sturdy design tells us that it is created by design people and not hardcore electronics guys. You know how bad they are :P! The project witnessed steady rise in its popularity and today it has become the most popular prototyping tool.
Arduino Circuits: As of writing this there are 58 different types of Arduino circuits (simply referred to as Arduinos). You can choose from this variety of circuits the one which fits your application needs. The most popular of them is Arduino Uno.
Arduino IDE: This piece of software helps you write application logic with a fancy editor, compile it and burn it on the circuit board. It can be installed on Windows, Linux and Mac OS platforms. There is also a web based IDE (if you just want to try).
Arduino Programming Language: Arduino programs are called sketches. A sketch is divided into two parts setup() and loop(). Numerous built-in functions are available to make the job easy. The language is largely inspired by C, try to see the code displayed in IDE!
Arduino Culture and Community: Arduino has a strong community helping each other on forums and sharing project ideas. The official website itself is a great resource for learning basics. Arduino day is a world-wide event celebrated in April each year that brings together developers, enthusiasts, students, teachers and novices.