Arduino is well known and well recommended open source microcontroller platform in the world. If you are starting with this you will definitely need to go through basics to start working with it. Finding good tutorial is essential to make successful projects as fast as possible. Tronixstuff has prepared a nice Arduino tutorial which will lead you from basics of it to more advanced techniques that helps to make great designs.
What differs this tutorial from others? Probably illustrations – lots of them. Every action is followed by illustration or video material where you can check visually your actions. Sometimes reading only can get you confused especially if you are new to these things. There are only thirteen chapters so far and seems that they are actively updated.