The image below shows an Arduino Uno Rev 3 board. It is a microcontroller board based on the ATmega328 microcontroller (the large integrated circuit you see in the image).
The Arduino can be used for building small circuits using electrical components. These components are connected to the Arduino via the digital and analogue input/output pins and controlled via programs (called sketches) which you upload to the microcontroller on the board.
Many of the projects in this section of the site can be built using components in the Sparkfun Inventor's Kit which comes with an Arduino Uno Rev 3 and a good selection of components to mess around with.
All of the diagrams showing connections between the Arduino and a breadboard have been made using the open source program, Fritzing. You can download the program from the Fritzing Web Site.