system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is an Arduino and how does it work?

Also, why is it used in what seems to be 90% of home-made electronics projects?
gpt
Okay... like you're five..

An Arduino is a small device called a micro-controller that is like a very small computer that allows programs to talk to and control other devices that do things like move stuff or measure things. Examples of things it could control: Motors, Levers, Lights, Speakers. It can talk to other things too, like: sensors such as thermometers, light meters, cameras and so on. The programs that do the thinking for the Arduino are created on a personal computer, and then generally stored on memory attached to the Arduino. If you want to make a program physically move something or interact with something, chances are you want to use a micro-controller like the Arduino.

You could use an Arduino to make a small robot like a car. Or you could use it to make a spy sensor for your bedroom door. Or if you really wanted to get crazy you could make a robotic nerf gun turret to shoot your sister when she gets into your stuff.

EDIT: tchebb is correct, Arduinos have onboard memory, and don't use memory cards.