Raspberry Pi is the favorite small hobby computer that can also be made into real-world projects and has been evolving into better configurations and versions of itself.
The original Raspberry Pi unit was based on the Broad-com BCM2835 and included a ARM 700 MHz processor with 256 Mb of Ram.
The newest version Raspberry Pi 2 model B uses a 900 MHz quad core ARM CPU and 1 GB RAM.
Even though the hardware has been updated for the Raspberry Pi from its initial release much like a PC 8086 based processors they are made to be backwards compatible. So code written for a project on an original Raspberry Pi will work on any later additions, including the newest.
As the Raspberry Pi has evolved it also has added on additional ports since its original variation such as 2 more USB slots. The extra USB slots are used for external devices such as Raspberry Pi compatible wireless adapters / dongles.
Raspberry Pi primarily uses Linux as an operating system, but the newer model Raspberry Pi 2 will be able to run Windows 10. Also other favorite operating systems include Kodi (formerly XBMC), FreeBSD, and Kali Linux among others.
Here are the Raspberry Pi models oldest to newest and their specifications.
Raspberry Pi Model A+ (256MB)
Raspberry Pi Model B 756-8308 Motherboard (RASPBRRYPCBA512)
Raspberry Pi Model B+ (B PLUS) 512MB Computer Board