The Arduino Uno is the flagship Arduino Hardware prototyping platform, and is the device that you will use the most throughout the Exploring Arduino book. The MCU on the Uno is the ATmega328. The board has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB-B connection, a power jack, an ICSP header, and a reset button. You can learn more about the specifications of the Arduino Uno here. There are many compatible clones for the Arduino Uno available, including the Adafruit METRO 328. You can use compatible clones interchangeably with the Uno for all the examples in Exploring Arduino.
Buy the Arduino Uno from Amazon, Arduino, Newark, or SparkFun.