Embedded development platform
Arduino is both hardware and open-source software giving the opportunity to discover, use and interact with the analogical and numeric world at relatively low cost.
The Arduino open-source library (often mistakenly named O/S) gives a level of abstraction which make learning and development easier and faster.
Company like microchip are selling clone of the arduino platform named ChipKit based on their pic32 MIPS core.
The site notice says the circuit board designs are all rights reserved, seemingly in contradiction of The Power of Open
For the software: read license for more details.