Reads incoming serial data.
Serial.read()
Arduino Mega only: 
Serial1.read() 
Serial2.read() 
Serial3.read()
None
the first byte of incoming serial data available (or -1 if no data is available) - int
int incomingByte = 0;	// for incoming serial data
void setup() {
	Serial.begin(9600);	// opens serial port, sets data rate to 9600 bps
}
void loop() {
	// send data only when you receive data:
	if (Serial.available() > 0) {
		// read the incoming byte:
		incomingByte = Serial.read();
		// say what you got:
		Serial.print("I received: ");
		Serial.println(incomingByte, DEC);
	}
}