An Arduino is a small computer you can hold in your hand. Its 'Brain' is a Microcontroller chip which is cheap and designed for making interesting high-tech things. We call Arduino a "MicroComputer". You can buy one for $30 or less from many different companies, including YourDuino.com.
There are many different versions of Arduino because it is "Open Source Hardware" which means that all the design details are made available by each manufacturer. The "Arduino Uno" shown above is from the originators (http://arduino.cc). Here's what they say Arduino is:
Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments.
All Arduino-compatible Microcomputers have the same physical shape so that accessories called "Shields" can plug in on top of them.
A couple more "Arduinos":
So, you probably have a few things to learn about Arduino and Electronics.
We will LEARN BY DOING (click)