I was shopping for hard drives and realized that I never understood the difference between SATA and SAS. I knew enough to know that Serial Attached SCSI (SAS) is an extension of Serial ATA (SATA) intended for servers.  So, SAS is to SCSI what SATA is to the retroactively named PATA.

While both are hot-swappable, SAS extends SATA by adding a unified power and data connector which looks similar but allows for a single plug.  A single interface allows for hot-pluggable mounts. The main difference is this connector.  This Tom’s Hardware Article has a good picture of the difference between the connectors.

Other differences include the SAS specification starts at the current max speed of SATA, 3.0 GB/s.  SAS also allows for data transfer at rates of up to 12.0 GB/s.  Typically you can buy SAS drives in traditional SCSI sizes and faster spin rates of up to 15,000 RPM.