What is an Arduino ?
If you have ever wanted a small gadget to control something with, Arduino is the answer.
The Arduino team have taken a microcontroller which is basically a single chip computer and built an easy to use physical computing platform around it.
You already have countless microcontrollers in your life, there is one in your washing machine for instance. On thier own microcontrollers can be very difficult to use, difficult enough to be beyond the access of all but the most dedicated hobbyists. What the Arduino team have done is produce a design which reduces the learning curve to the absolute minimum while still providing a powerful platform for project development.
What is most impressive about the arduino is the range of sensors available, my own interest is in measuring the effect of setup changes on my radio controlled cars and also in providing active in car control. There are off the shelf boards and software that literally allow the sensors I need to be plugged in to the Arduino with no need to seperatley source individual electronic components or even get deep into understanding thier purpose.
Its a bit like building a PC, if I need a graphics card, I buy a graphics card, I don't need to buy 300 components or even understand what they do, I just buy and plugin a ready made solution which does what I want.
Arduino does what I want.