Arduino is an open-source computer hardware and software company, project and user community that designs and manufactures kits for building digital devices and interactive objects that can sense and control the physical world.
This is a good starting point for those unfamiliar Arduino. The tutorials explains basic such as:

  • What is arduino
  • What you can do with Arduino
  • Different t of Arduinos.
This link explains how to put code on an Arduino and fixes some of the basic problems, a person who has never coded before on Arduino might encounter.
The differences between the many different sorts of Arduino and what they can be used for. Also explains some of the common terms used, very good reading for those who have never used Arduinos before.
Introduction to the sorts of data / programing terms encountered with using an Arduino. If you are planning on using the Arduino and you have not programmed with one before, this is a good reference list to come back to.
This is a very good explanation listing what Shields are and how they work with Arduinos.
Introduction to Arduino and starting from the beginning. Perfect introduction for people who have never used an Arduino before.

The Digital Sandbox (DS) is a learning platform that engages both the software and hardware worlds. It’s powered by a microcontroller that can interact with real-world inputs – like light or temperature sensors – while at the same time controlling LEDs, motors, and other outputs. The Digital Sandbox is equipped with everything, on board, that you will need to complete 13 experiments including controlling an LED, measuring how loud things are, detecting what the temperature is, and more.
Intro to what the Digital Sandbox is and what it is used for. 
List of tutorials for the Sandbox.

MaKey MaKey is an invention kit for the 21st century. Turn everyday objects into touchpads and combine them with the internet. It’s a simple Invention Kit for Beginners and Experts doing art, engineering, and everything in-between:
This site explains Makey Makey and outlines how the kits work and the possibilities available.

General Arduino tutorials. If you have a specific part that you would like to learn about it; find it here. These tutorials range from introduction level to advance, so only pick ones you are comfortable with.
Full list of lessons in relation to Arduinos on the Adafruit website.
Basic introduction to Makey Makey. Great starting place for those who have never used it before

Australian based Electronics Company, stocks most versions of Arduino and also has electronic kits.