Arduino is an open-source prototyping platform based on easy-to-use hardware and software. Arduino boards are able to read inputs - light on a sensor, a finger on a button, or a Twitter message - and turn 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 set of instructions to the microcontroller on the board. To do so you use the Arduino programming language (based on Wiring), and the Arduino Software (IDE), based on Processing.
That actually says it all.
You can find the complete introduction here.
Maybe a little more information about the board:
The Arduino Board itself is a blue circuit board, the size of a credit card (but they also have models in other sizes). It has two rows of connectors (the 'headers'), a power connector and a USB connector. The brain of the board is an Atmel microcontroller. It's like a really small, very low power 'computer'. (It only has 32KB of storage, 2KB of RAM, and the 8-bit processor runs at only 16MHz.) For most applications, however, this is more than enough. The pins of the processor connect to the headers, and you can connect them to virtually anything you can imagine. You just need to write some lines of code to control them. The I/O pins (Input/Output) can be used as input pins, to connect buttons or knobs, temperature sensors, light sensors, or even keyboards and mouses, digital musical instruments ... or they can be used as output pins, to connect LEDs, drive motors, control the lights in your home, connect to small displays or even connect to the Internet, so that it can check your mail, post tweets ... Through the USB connection you can also use it to control your computer, or use your computer to control the Arduino.
As you can see, the sky's pretty much the limit !
Arduino is a popular open-source development board for engineers and makers to develop electronics projects in an easy way. It consists of both a physical programmable development board (based on AVR series of microcontrollers) and a piece of software or IDE which runs on your computer and used to write and upload the code to the microcontroller board.
Explore below some interesting arduino projects and tutorials based on different types of arduino baords like Arduino Uno, Arduino Pro Mini, etc. These DIY arduino projects for beginners are explained well and you can find the complete guide to DIY these projects with the help of circuit diagrams, source codes and videos. You can also browse below pages to get new arduino project ideas.