What exactly is an Arduino?

Arduino is an open source physical computing platform   based   on   a simple   input/output (I/O) board and an integrated development environment (IDE) that implements   the   Processing   language.

                                  In its simplest form, an Arduino is a tiny computer that you can program to process inputs and outputs. For example, a  simple use of the Arduino would be to turn  a  light  on  for  a  set  period  of  time,  let’s  say  30 seconds,  after  a  button  has  been  pressed  The Arduino would sit  patiently waiting for  the button  to be pressed. When  you  press the  button  it  would  then  turn  the  lamp   on  and  start counting.  Once  it  had  counted  30  seconds  it  would then  turn  the  lamp  off and then  carry  on  sitting there waiting  for  another  button  press.  You  could  use  this set-up  to  control  a  lamp  in  an  under-stairs,cupboard,porch etc.. for  example.  You  could extend this example to sense when  the  cupboard  door  was  opened  and automatically  turn the light on, turning it off after a set period of time. The  Arduino  can  be  used  to  develop   stand-alone interactive  objects  or  it  can  be  connected  to  a computer  to  retrieve or  send  data  to the Arduino and then act on  that data  (e.g. Send sensor data  out to the internet).The Arduino  can  be  connected  to  LEDs.  Dot  Matrix displays,  LED  displays,  buttons, switches,  motors, temperature  sensors,  pressure  sensors,  distance sensors,  webcams,  printers,  GPS receivers,  Ethernet  modules, etc..etc..

                                     The  Arduino  hardware  and  software   are  both  "Open Source",  which  means  the  code,  the  schematics, design, etc. are all open  for  anyone to take freely and do what they like with it.