The Raspberry Pi is much more powerful than an Arduino but it has a few limitations of its own. Its I/O is much more limited, it can't use common Arduino shields, and it needs a regulated 5V power supply via its USB connection. All those limitations are solved with the PiLeven.
The Freetronics PiLeven is an Arduino-compatible board based on the Arduino Uno, but with a few changes. Obviously it's a bit of a strange shape! The PiLeven fits right on top of a Raspberry Pi (either model B or B+) using the Raspberry Pi expansion headers. The PiLeven also has a high-current switchmode power supply, so you can plug in anything from 7 to 18 V DC using a standard 2.1 mm jack. The PiLeven can power the Raspberry Pi, so you don't need a regulated 5V USB connection anymore.
Serial communications on the PiLeven is linked through to the Raspberry Pi, so your Pi can upload new sketches straight to the PiLeven or send/receive data and commands. Also included are level shifters so the 3.3V Pi can talk safely to the 5V PiLeven. You can plug standard Arduino shields right into the PiLeven, giving your Raspberry Pi access to the huge range of shields already available.