Digital, in the computer and electronic worlds, means discrete, which is the opposite of analog/continuous. It is also a mathematical definition. We often talk about domains to define the cases for use of digital and analog.

Usually, the analog domain is the domain related to physical measures. Our temperature can have all the values that are possible and that exist, even if our measuring equipment dosen't have an infinite resolution.

The digital domain is the one of computers. Because of the encoding and finite memory size, computers translates analog/continuous values into digital representations.

On a graph, this could be visualized as follows:

Arduino owns inputs and outputs. We can also distinguish analog and digital pins.

You have to remember the following points:

Inputs and outputs are pins provided by the board to communicate with external peripherals.

We often talk about reading pins for inputs and writing pins for outputs. Indeed, from the Arduino board point of view, we are reading from the world and writing to the world, aren't we?

A digital input is a digital pin set up like an input and providing the capacity for electrical potential reading and conversion to 0 or 1 to the Arduino board. We'll illustrate this very soon using switches.

But before manipulating this directly, let me introduce a new friend named Processing. We'll use it to easily illustrate our Arduino tests further in the book.