SCSI stands for Small Computer Systems Interface and is basically a connection standard for all kinds of devices such as Hard Disks, Tape Drives and CD-ROM's.
It uses components that are different from IDE.
SCSI has different standards, the biggest differences are the Data Bus width and the maximum transfer speed.
The Data Bus can be 8 or 16 bits wide, the transfer speed can go up to as fast as 320MB/s.
There are many different cables and connectors for different SCSI standards but the most commonly used ones are either 68 pin ( U3W ) or 80 pin ( SCA ).