If you don't know what an Arduino is, its basically your computer, except extremely tiny.
Arduino is an open-source hardware and software company, project and user community that designs and manufactures single-board microcontrollers and microcontroller kits for building digital devices. (credit to Arduino)
Arduinos can be used in lots of different things, you can build a rocket that uses an Arduino Computer for its entire guidance system, and stabilization system. You can also use that tiny little computer, no we aren't talking about Arduinos (that's a joke if you didn't know!), to be in a car, providing autosteer, autodrive, anti-collision system, and more. With Arduino's, you can literally make anything possible. Arduino's of course, are a bit less powerful than a full-size PC with a case and everything, but an Arduino is still pretty powerful for its size. Raspberry Pi's are also pretty similar to Arduino's, except Arduino's are usually relatively cheaper. Raspberry Pi's usually have more RAM than Arduino's, and have better CPU's, but Arduino's are cheaper as mentioned before.
This blog post IS mainly about Arduino's, but if you would like to make a project using an Arduino, you might want to use a Raspberry Pi instead, as it is more powerful, and could potentially be more useful to bigger projects.