Parallel SCSI (Small Computer System Interface) is a standard for connecting and transferring data between computers and peripheral devices, such as hard drives and scanners. It uses multiple data lines to send and receive information simultaneously, which allows for faster data transfer rates compared to older methods. This technology was widely used in the 1980s and 1990s, particularly in servers and workstations. However, as technology advanced, Serial SCSI and other interfaces like USB and SATA became more popular due to their improved performance and simpler connections.