Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments.
First thing: get yourself an Arduino board from one of the popular online electronic shops like Sparkfun or some in India such as Robokits India and Rhydolabz.
One may use avr-gcc and avrdude if they choose to program in C using barebone Makefiles, but the easiest way to program an Arduino board is using the Arduino IDE that is supported on Linux, Mac OSX and Windows. Once you’ve the IDE and the board setup, get some LEDs or perhaps servos or sensors and play around the tutorials listed on the project webpage.
The Arduino IDE is written in Java and based on Processing and mainly uses avr-gcc, avr-dude. A typical Arduino sketch is written in Wiring, an open source electronics prototyping platform derived from Processing which has a simplified C++ language, an IDE and used for a single board microcontroller. In a typical sketch a programmer is only required to define two functions; setup() and loop(). When you compile your Arduino sketch, it simply generates equivalent AVR-C code and compiles it into an Intel hex file which is uploaded to the Arduino’s microcontroller’s on-chip flash memory by avr-dude. Many Arduino hackers use Fritzing, a project that aims at providing tools for designing and prototyping the hardware, pcb layout. Now start hacking Arduino Like a Boss™.