Above is a picture of the ATmega328 silicon die/chip. This is what's inside that large, long IC on many Arduino UNO compatibles and the small square IC on an Adafruit Metro Classic (below) and on an Arduino pro mini. Both packages appear on a number of other comparable microcontroller boards.
In this guide we'll take a tour of the '328, looking at each significant functional piece. This guide is going to be fairly high level, most of the concepts will be applicable to any microcontroller. For full details on the '328 see the datasheet from Microchip (note it is very long, technical, and sleep inducing).