Reference → Serial →

Serial.begin()

Sets the data rate in bits per second (baud) for serial data transmission. For USB baudrate is fixed on 115200 bps, on UART it is possible to select baudrate. Sets the data rate in bits per second (baud) for serial data transmission. For communicating with Serial Monitor, make sure to use one of the baud rates listed in the menu at the bottom right corner of its screen. You can, however, specify other rates.
 An optional second argument configures the number of data, parity, and stop bits. The default is 8 data bits, no parity, one stop bit. 

Serial.begin(baudrate) // for UART
Serial.begin()         // for USB-CDC
Serial.begin()
Serial.begin(speed)
Serial.begin(speed, config)
Serial.begin(speed, config, rx, tx) - returns error code.
Serial.begin(speed, config, rx, tx, *buffer, len) - returns error code
 baudrate baudrate to be used. Supported values are: 9600, 14400, 19200, 38400, 57600, 115200, 230400 bps. If omitted, default 115200 bps is used (the only available on USB interface). For lower baudrates use Software Serial speed: in bits per second (baud). Allowed data types: long. Standart supported values are: 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, 115200, 230400 bps. If omitted, default 115200 bps is used. config: sets data, parity, and stop bits. Valid values are:
  • SERIAL_8N1 (the default)
  • SERIAL_8N2
  • SERIAL_9N1
  • SERIAL_9N2
  • SERIAL_8E1: even parity
  • SERIAL_8E2
  • SERIAL_9E1
  • SERIAL_9E2
  • SERIAL_8O1: odd parity
  • SERIAL_8O2
  • SERIAL_9O1
  • SERIAL_9O2
 rx: pin RX. tx: pin TX. buffer: pointer to receiving buffer. len: size of buffer in bytes. nothing error code. 0 means success. Simple variants of begin() method. 

    	Serial.begin();      // opens serial port on USB, sets data rate to 115200 bps
    	Serial1.begin(9600); // opens serial port on UART1, sets data rate to 9600 bps
Advanced variants of begin() method. 

    static uint8_t buf[256]; // Custom receiving buffer
    Serial0.begin(115200, SERIAL_8N1);//  Setup just a bit mode. 8 bita + 1 stopbit
    Serial0.begin(115200, SERIAL_8N1, 9, 10);// Define bit mode and move serial0 to pins 9/10
    Serial0.begin(115200, SERIAL_8N1, 9, 10, &buf[0], sizeof(buf));// Define bit mode, move serial0 to pins 9/10 and use custom receiving buffer