The beauty of the Arduino platform is undeniably its accessibility: you don't have to have a background in electronics or circuit design to get started, and with many board models you can begin flashing an LED with absolutely zero additional components. After a certain point, though, knowing what's going on under the hood makes your life significantly easier - especially if you're moving on from pre-built Arduino boards and into the realm of self-built Arduino compatible circuits - and that's exactly why Yahya Tawil has written a comprehensive guide to the inner workings of the Arduino Uno.
Yahya's write-up of the Uno's design serves two distinct purposes: written to be accessible for beginners it acts as a great introduction to some of the technical terms and technologies with which you'll need to be familiar if you're looking to delve deeper than just connecting wires to a breadboard, and it also serves as a detailed looked at the bits and pieces - not just the microcontroller itself - that go into the most popular Arduino design of all.
It's certainly not something you need to read if all you want to do is hook components up to an Arduino or compatible and make it do something, but for anyone who is curious or embarking upon the first steps of a hobby or career in circuit design Yahya's work can serve as a fantastic introduction.
Most articles explain the software of Arduinos. However, understanding hardware design helps you to make the next step in the Arduino journey. A good grasp of the electronic design of your Arduino hardware will help you learn how to embed an Arduino in the design of a final product, including what to keep and what to omit from your original design.