Raspberry Pi is a Miniature Computer that can Interface with a Display Device like TV and a keyboard.
Model A has been redesigned to have 256Mb RAM, one USB port and no Ethernet (network connection). Model B has 256Mb RAM, 2 USB port and an Ethernet port.
The Raspberry Pi measures 85.60mm x 53.98mm x 17mm, with a little overlap for the SD card and connectors which project over the edges. It weighs 45g.
The SoC is a Broadcom BCM2835. This contains an ARM1176JZFS, with floating point, running at 700Mhz, and a Videocore 4 GPU. The GPU is capable of BluRay quality playback, using H.264 at 40MBits/s. It has a fast 3D core accessed using the supplied OpenGL ES2.0 and OpenVG libraries.
Overall real world performance is something like a 300MHz Pentium 2, only with much, much swankier graphics.
There is composite and HDMI out on the board, so you can hook it up to an old analogue TV, to a digital TV or to a DVI monitor. No VGA support.