The kit consists of an Arduino shield (the Twig), a set of peripherals (the Leaves), and a set of cables that you use to link them up.
There are many other leaves available, and they can all connected to the motherboard using the cables provided. The documentation is sparse, but usable; we'll do our best to add to it.
At present the Grove is targeted at the Arduino, but we're looking at the possibility of connecting it to the Raspberry Pi. It's a natural fit with our planned extension board, and would offer an excellent platform for experimenters wanting to explore the Internet of Things.