Raspberry Pi is a credit card sized computer developed by the Raspberry Pi Foundation in the UK. It is based around a Broadcom BCM2835 SoC, which includes an ARM1176JZF-S 700 MHz processor, VideoCore IV GPU and has 256 Megabytes of RAM. There are a number of interfaces including HDMI for video output, an Ethernet connection, 2 USB ports, audio port, composite video connection and an SD card slot.
General purpose I/O pins allow you to connect it to other electronic compnents, and there are digital camera and lcd interfaces. It is powered from a micro-usb adapter.
The system boots from an SD card which contains the OS within about 15 seconds.
One goal is to encourage people to learn how to program and the Scratch Programming system from MIT is used as an example.
(video link: https://www.youtube.com/watch?v=6BbufUp_HNs )
It's available on sale from today, although it may be some time before you get your hands on one - the expected delivery date for mine is 16 April