The Raspberry Pi is a series of credit card-sized single-board computers developed in the United Kingdom by the Raspberry Pi Foundation with the intent to promote the teaching of basic computer science in schools and developing countries. - Wikipedia

Below you can see the different Raspberry Pi models organised in chronological order of release date. 

Raspberry Pi Model b (First Generation)

The first generation (Pi 1) was released in February 2012, costing a mere $35 and featuring:

  • 2 USB Sockets
  • 3.5mm Audio Jack
  • Composite Video
  • 26 Pin GPIO Header
  • Ethernet Port
  • HDMI Out
  • 512MB RAM
  • 700Mhz Single-Core CPU

 

Raspberry Pi model a

Released in February 2013, a year later, the Model A was designed to be smaller so to be better embedded into projects and to have a far lower power consumption. I did not feature an Ethernet socket and only one USB port was supplied, as well as the RAM being decreased to 256MB. $25

Raspberry Pi compute module

The Compute Module, released in April 2014 was designed for industrial applications within a more flexible form factor. The RAM contained increased back to 512MB, and could fit into  a standard DDR2 SODIMM connector. $30

Raspberry pi model b+

Arguably the biggest change for the consumer edition of the Raspberry Pi, July 2014 brought the B+, which had upgraded to a larger 40 pin GPIO header and featured 4 USB ports (up from 2). Whilst the RAM and CPU remained the same, the standard size SD slot was upgraded to a Micro SD slot which remains the same as used today. Power consumption improved effeciency as well as the Pi featuring more powerful audio processing and improved mounting holes that are now HAT compliant. The composite video output was removed and the sockets were arranged ergonomically to be on only two sides of the board. $25 

Raspberry pi model a+

In November 2013, the Raspberry Pi foundation announced the Model A+, which was the smallest standalone Pi yet. It contained the new 40 pin GPIO header and mounting holes. It featured the same 512MB RAM as all previous models (except model A) as well as the 700MHz single-core CPU. $20

raspberry pi 2 model b 

With an identical form-factor to the previous model B, the Raspberry Pi 2 announced in February 2015 featured a much overdue increase in processing power. The board updated its CPU to a 900MHz quad-core chip and gained an increase to 1GB of RAM. $35

raspberry pi zero

Undoubtedly the biggest and most exciting change to the Raspberry Pi, the Raspberry Pi Zero was released in November 2015 and was distributed for free with the Raspberry Pi Foundation's official magazine, the MagPi. The Pi Zero's form factor decreased to almost a quarter of the original Pi's size, and now featured an optional (solder yourself) GPIO header, 1GHz single-core CPU and 512MB of RAM. The Zero also could be connected to a display via the smaller Mini-HDMI output and to USB through a Micro-USB socket. The Zero used a fraction of the power as any other model of Raspberry Pi to date. Oh, and it costs £4! ($5)

raspberry pi 3 model b

February 2016 brought the Raspberry Pi 3, now more powerful and connectable than ever. On board, the Pi 3 featured a whopping 1.2 GHz quad-core CPU (64 bit) and 1GB of RAM, as before. The biggest change for the Pi 3 however was the built in WiFi and Bluetooth connectivity allowing the Pi to be better integrated into IoT (Internet of Things) projects without the need for bulky adaptors.

Raspberry pi zero v1.3

With no changes to the computational abilities or form factor, the new Pi Zero released in May 2016 now featured a camera connector, which is smaller than previous camera connectors meaning that a new camera cable is required.  

Further to actual Raspberry Pi computers, the Raspberry Pi Foundation has also released the following official products: