This tutorial shows you how to build an Arduino on a breadboard. You would do this to build a circuit before you finalize it on a PCB. It discusses the power setup that you need for the project and how you program it when the processor is in circuit.
It describes the use of 16MHz and 20Mhz crystals for driving the Arduino and compares their use to the use of a resonator. A simple circuit is created that blinks an LED for 1000ms and 100ms. The Arduino is programmed on the breadboard using an Arduino shield’s Reset, TX and RX pins.