A Raspberry Pi is a credit card-sized computer originally designed for education, inspired by the 1981 BBC Micro. Creator Eben Upton’s goal was to create a low-cost device that would improve programming skills and hardware understanding at the pre-university level. But thanks to its small size and accessible price, it was quickly adopted by tinkerers, makers, and electronics enthusiasts for projects that require more than a basic microcontroller (such as Arduino devices).
The Raspberry Pi is slower than a modern laptop or desktop but is still a complete Linux computer and can provide all the expected abilities that implies, at a low-power consumption level.
The Raspberry Pi is open hardware, with the exception of the primary chip on the Raspberry Pi, the Broadcom SoC (System on a Chip), which runs many of the main components of the board–CPU, graphics, memory, the USB controller, etc. Many of the projects made with a Raspberry Pi are open and well-documented as well and are things you can build and modify yourself.
These are the models of the Raspberry Pi which are currently available: the Pi 3 Model B, the Pi 2 Model B, the Pi Zero, and the Pi 1 Model B+ and A+.
The Model A+ is the low-cost variant of the Raspberry Pi. It has 512MB RAM (as of August 2016: earlier models have 256MB), one USB port, 40 GPIO pins, and no Ethernet port. The Model B+ is the final revision of the original Raspberry Pi. It has 512MB RAM, four USB ports, 40 GPIO pins, and an Ethernet port. In February 2015, it was superseded by the Pi 2 Model B, the second generation of the Raspberry Pi. The Pi 2 shares many specs with the Pi 1 B+, but it uses a 900MHz quad-core ARM Cortex-A7 CPU and has 1GB RAM. The Pi 2 is completely compatible with first generation boards, and is the model we recommend for use in schools, due to its flexibility for the learner. The Pi 3 Model B was launched in February 2016; it uses a 1.2GHz 64-bit quad-core ARM Cortex-A53 CPU, has 1GB RAM, integrated 802.11n wireless LAN, and Bluetooth 4.1. Finally, the Pi Zero is half the size of a Model A+, with a 1Ghz single-core CPU and 512MB RAM, and mini-HDMI and USB On-The-Go ports.
You can check Raspberry Pi products page for more details on current boards. There are also some models of Raspberry Pi which are no longer in production, but which may be available second-hand or from resellers. The Model A was the initial low-cost variant of the Pi. It was replaced by the smaller, neater Model A+ in November 2014; it shares the same specs as the A+, but has only 26 GPIO pins. The Model B was the previous incarnation of the B+; again, it shares most of the same specs, but has only 2 USB ports and 26 GPIO pins.
The Raspberry Pi was designed for the Linux operating system, and many Linux distributions now have a version optimized for the Raspberry Pi.
Two of the most popular options are Raspbian, which is based on the Debian operating system, and Pidora, which is based on the Fedora operating system. For beginners, either of these two work well; which one you choose to use is a matter of personal preference. A good practice might be to go with the one which most closely resembles an operating system you’re familiar with, in either a desktop or server environment.
If you would like to experiment with multiple Linux distributions and aren’t sure which one you want, or you just want an easier experience in case something goes wrong, try NOOBS, which stands for New Out Of Box Software. When you first boot from the SD card, you will be given a menu with multiple distributions (including Raspbian and Pidora) to choose from. If you decide to try a different one, or if something goes wrong with your system, you simply hold the Shift key at boot to return to this menu and start over.
There are, of course, lots of other choices. OpenELEC and RaspBMC are both operating system distributions based on Linux that are targeted towards using the Raspberry Pi as a media center. There are also non-Linux systems, like RISC OS, which run on the Pi. Some enthusiasts have even used the Raspberry Pi to learn about operating systems by designing their own.