As you can see in the image above the Arduino is available in many layouts, colors and features. The basic entry level board would be the Arduino Uno. From there we have more feature rich boards such as the Arduino YUN and EtherTen shown above which have Ethernet functionality built in. From there we go to the Arduino Zero using the Cortex CPU (32bit) and the Arduino Due which uses the ARM CPU platform.
There are many other Arduino boards such as the Arduino Micro and Mini but we do not cover those boards.
The latest boards to be released are the Arduino 101 (US) and Genuino 101 (EU, CA, Middle East). This board uses the Intel Curie module that contains an x86(Quark) and a 32bit ARC architecture core both running at 32MHz.
Click on any of the Arduino images below for more details on each board