The credit-card single board computer (SBC) launched in in the UK by a charity the Raspberry Pi Foundation available from April - June 2012 - Raspberry Pi Model B costing only £22 / $35 was available to buy online from various suppliers.
Since then, a revised version was released October 2012 with more on-board (512MB vs. 256MB) memory SDRAM.
The smaller sister version without onboard ethernet the Model A was launched February 2013.
Then in the summer of 2014 (July) the Raspberry Pi Model B+ came out with extra USB ports (4 in total), followed by the Raspberry Pi Compute Module in April 2014 and then the Raspberry Pi Model A+ in November 2014,
In February 2015 the Raspberry Pi 2 Model B a faster cpu (900Mhz) and bumped up the onboard memory to 1GB of SDRAM still for only £22/$35.
In February 2016 the Raspberry Pi 3 Model B a faster cpu (1.2Ghz) and for the first time included built)in Wifi (802.11n) and Bluetooth 4.1
You can read more on the official blog http://www.raspberrypi.org/blog/