SCSI, PS/2 & USB are computer ports whereas POP3 is a protocol.POP3It is a client/server protocol in which e-mail is received and held for you by your Internet server.Periodically, you (or your client e-mail receiver) check your mail-box on the server and download any mail, probably using POP3.Important PointSCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices.PS/2 It is a 6-pin mini-DIN connector used for connecting keyboards and mice to a PC compatible computer system. USBIt is a standard cable connection interface for personal computers and consumer electronics devices. USB stands for Universal Serial Bus, an industry standard for short-distance digital data communications.