If you have Arduino boards with a single serial port (Uno, Nano, mini) you can use SoftwareSerial, http://arduino.cc/en/Reference/SoftwareSerial
Create a TX and RX on both boards with SoftwareSerial, and connect TX to RX, and RX to TX. Don't use pin 0 or 1 (RX and TX for serial monitor and upload), and maybe also not 13(led). That's all.
If you have a Mega, you have more serial ports, Serial1 and more.
If you have a Leonardo or Micro, use the Serial1 port on pin 0 and 1.
For the Mega, Leonardo and Micro, use the standard Serial library, http://arduino.cc/en/Reference/serial