Raspberry Pi is a Credit-Card size computer
The Raspberry Pi is a credit card-sized computer that plugs into your TV and a keyboard. It is a capable little computer which can be used in electronics projects, and for many of the things that your desktop PC does, like spreadsheets, word processing, browsing the internet, and playing games. It also plays high-definition video. We want to see it being used by kids all over the world to learn programming.
These are the current models of the Raspberry Pi available: the Pi 2 Model B, the Pi 3 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 256MB RAM, 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 (twice as much as the A+), 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 In February 2015, it was superseded by the Pi 2 Model B, the second generation of the Raspberry Pi.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.
More details can be found at Raspberry Pi Official Page.