Sets the data rate in bits per second (baud) for serial data transmission. For communicating with the computer, use one of these rates: 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, or 115200. You can, however, specify other rates - for example, to communicate over pins 0 and 1 with a component that requires a particular baud rate.
Serial.begin(speed)
Arduino Mega only:
Serial1.begin(speed)
Serial2.begin(speed)
Serial3.begin(speed)
speed: in bits per second (baud) - long
nothing
Arduino Mega example:
Thanks to Jeff Gray for the mega example