The Arduino UNO board layout.

• The Power LED indicates that the board has power. 
• The TX and RX LEDs indicate serial communications traffic. 
• TX is blinking when transmitting, and RX is blinking when receiving. 
• The Pin 13 LED is connected to the Arduino Digital I/O pin 13 and can be used for a variety of software tests without having to add any external hardware to see the software in action.

Arduino Uno Rev3——–>

How an Arduino Program Works———->

• What is Arduino?
• Why Arduino?
• How do I use Arduino?