"USB is also a serial communication method" is this true?
Most wired communication nowadyas is serial, and so is USB. Serial just means that bits are transferred one after another in time, so that you only need one wire (two if you want to do it differential, like in USB). Expensive term: Time Division Multiplex, compared to Space Division Multiplex, where different bits use different physical wires in parallel. Remember the old IDE hard disks, floppy drives and SCSI devices for instance and the flat cables they used. 



Serial has the advantage of smaller and cheaper connectors and cable, but needs a much higher bandwidth than parallel. Nevertheless at high speeds also parallel communication may be timing sensitive if there are delay differences between the different lines, so that all signals may not arrive simultaneously.