Several generations of Raspberry Pis have been released.The first generation (retrospectively known as the Raspberry Pi 1) was released in February 2012 in basic Model A and a higher specification Model B. Improved A+ and B+ models were released a year later. The Raspberry Pi 2 was released in February 2015 and Raspberry Pi 3 in February 2016.
Here a picture of the Raspberry 3:
The Foundation provides Raspbian, a Debian-based Linux distribution for download. This Raspbian is the base system for the images we picochess developers provide on Picochess Images. The images are btw. based on Jessie.