Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the print() function instead.
Arduino Mega also supports:
Serial3 (in place of
val: a value to send as a single byte
str: a string to send as a series of bytes
buf: an array to send as a series of bytes
write() will return the number of bytes written, though reading that number is optional