Arduino Introduction

Arduino is an open-source electronics platform based on easy-to-use hardware and software.
Arduino boards are able to read inputs - light on a sensor, a finger on a button, or a Twitter message - and turn it into an output - activating a motor, turning on an LED, publishing something online.
You can tell your board what to do by sending a set of instructions to the microcontroller on the board. To do so you use the Arduino programming language (C++), and the Arduino Software (integrated development environment - IDE).
A worldwide community of makers - students, hobbyists, artists, programmers, and professionals - has gathered around this open-source platform, their contributions have added up to an incredible amount of accessible knowledge that can be of great help to novices and experts alike.

Over the years Arduino has been the brain of thousands of projects, from everyday objects to complex scientific instruments.
Inexpensive.
Cross-platform - works on Windows, Mac, Linux etc.
Simple, clear programming environment.
Open-source hardware empowering users to build them independently and eventually adapt them to their particular needs.
Software growing through the contributions of users worldwide.

Referenced from the perspective of the Arduino Board

Inputs is a signal / information going into the board Output is any signal exiting the board

Almost all systems that use physical computing will have some form of output
A device which can provide input(s) is called an input device, usually referred to as sensors. Ex: Light sensors (LDRs), Accelerometers, Push buttons
Output devices are usually referred to as actuators Ex: Motors, LEDs

Shields provide an easy way to interface sensor and actuators with the Arduino – avoids having to wire them up manually
Shields can be stacked (terms and conditions apply!)

Arduino