Small computer standard interface, drives typically used in servers as already stated. The drive controllers are based on cards rather than integrated into the drive itself (as in IDE, integrated drive electronics).
SCSI drives as they're aimed at enterprise users carry a large price premium but are generally more reliable, offer hot swap capability and higher rotational speeds (typically 15,000 RPM) they also usually require some form of active cooling and tend to have heatsinks forming part of the housing.