The Raspberry Pi is a credit card sized computer that is primarily designed to encourage a new generation into learning about computers and programming rather than simply relying on using pre-packaged products like Microsoft Word and pre-written games.
There are two models: the model A and the model B. The Model B has the following interfaces:
Ethernet
2 USB ports
HDMI
RCA Video
3.5mm stereo audio socket.
It also has a mini (or micro not sure which) USB port that is used solely to provide the power input from a power adapter similar to a phone charger. (Check the power requirements.) The Model B now includes 512 MB of RAM. The Pi boots from an SD memory card that plugs into a card slot on the underside of the board.
Keyboard and pointing device (mouse, track ball, etc) are connected via USB either directly or through a USB hub. External storage can also be connected through the USB ports.
The Pi also has some basic raw hardware connection headers to allow it to be connected to end user hardware to make such things as a robot, an automated camera system, an automated home management system, etc.
The model A has less memory, no Ethernet port and a single USB port.
The normal operating system is based on Linux. There are various distributions available. One distribution (Raspbmc) supports various Internet TV services.
I have an early model B, which only has 256 MB of RAM, which is connected to my TV for watching Internet video. I also have a later model B that is configured to dual boot. This one is connected to a computer monitor with an HDMI to DVI cable and computer speakers via the 3.5mm socket. This second machine is configured to dual boot either to watch Internet TV or to provide a software development type environment that the Pi was originally intended for. I have done some programming on it both in C and in Python.
I hope this information is useful.