What is arduino serial and how is it any different to the other boards?
What is the main reason for using one?
An arduino serial is an arduino board that has an RS232 serial input instead of a USB interface.
What is the main reason for using one
It is cheaper if you have an old computer with a serial port.
It is cheaper if you have a USB / RS232 lead and you make your own arduino boards.
What is the main reason for using one
Also, once uploaded and running a sketch, it can talk directly to "real" serial devices (while a USB arduino would need at least a TTL/RS232 converter.)