The Raspberry Pi measures roughly 3.4 inches by 2.1 inches (8.6 centimeters by 5.3 centimeters), but it is pretty powerful for such a small device. This was made possible by the ready availability of inexpensive and tiny processors for mobile devices, which need to pack a decent amount of processing and multimedia capability into a small shell with the ability to stay relatively cool and not suck power too quickly.
The foundation picked a chip with ARM architecture for this reason (a processor architecture commonly used for mobile phones and similar devices). The chip has 256 MB of RAM, runs at 700 MHz and includes a 1080p-capable GPU. Although there are other ARM chips available, the group chose a Broadcom chip in part because of Eben Upton's relationship with the company (he works for them). Broadcom's willingness to give a bulk rate for small orders allows the foundation to get a much better price on this chip than on any comparable competitor's processor.
Like many of the earliest home computers, the device comes without peripherals or internal storage space, and the user will have to attach input, output and storage peripherals. At a minimum, you'll need a television or monitor for output, a keyboard (and possibly a mouse) for input, an SD card on which to house the OS and store data, a power supply and any necessary cables. You can add an external hard drive for additional storage, but the SD card will still be necessary, as the OS will boot from SD by default.
The compatible operating systems for the device are all Linux distributions. Linux was chosen at least partially for its low memory overhead, making it possible to run a fully functional OS on such a simple device that's devoid of built-in permanent storage. Linux is also generally free and has great potential as a CS learning tool, since its distributions often come with some programming languages already installed.
The open-source nature of Linux will aid in the proliferation of software as developers jump on the bandwagon to provide content. The Raspberry Pi Foundation's initial intention was to create both the device and the learning curriculum around it, but the group decided to scale back its scope, concentrate on creating the computer and let an eager, willing open-source programming community create software.
The Raspberry Pi's programmability and simplicity make it very like the computers of yore that spawned so many programmers and system hobbyists. But unlike those computers, this device can be used for Web surfing. The Internet will make finding things you can do with the device much easier than back in the day. There's a user forum on the Raspberry Pi site, and tutorials and other materials are readily available online. Of course, either the Ethernet connector of Model B or an external WiFi device attached via USB will be necessary to allow for network connectivity.