SCSI or Small Computer Systems Interface is technology designed to connect devices to a computer. SCSI is a bus technology, which means that all devices connect to a central bus and are "daisy-chained" together. The SCSI bus is controlled by a host controller which will typically be built into the motherboard, or it can be from a separate expansion card.
A SCSI connector is either external or internal. The cabling/connector requirements depend upon the location of the SCSI bus.