Serial Attached SCSI is the newest revision of SCSI, intended to correct many of the mounting problems with older parallel SCSI systems. It uses connectors and cabling identical to those used by Serial ATA, and is indeed compatible with SATA devices. The inverse is not true, however - SAS devices can't be attached to a SATA bus. There are, however, quite a few differences between SATA and SAS. For one, SAS uses a higher signaling voltage, which allows for longer cables. SAS cables can be up to 8 meters long, compared to the 1m maximum for SATA. When SATA devices are attached to a SAS controller, all devices attached to that port switch down to use SATA signal voltages. This means that SATA drives work on a SAS bus, but in some cases, like server backplanes, they may not be freely interchangeable.
SAS also supports the full SCSI command set, meaning that in theory, SCSI Generic devices like scanners, printers, media changers and tape drives can be attached to a SAS bus. In practice this may be moot, since USB and Firewire have largely displaced SCSI in this role. SAS also supports longer cables, which means that external SAS devices won't have nearly as many constraints as eSATA devices.
Currently, most SAS controllers use an LSI Logic chipset, though Adaptec and 3Ware also produce them. Sun, HP and Dell offer SAS in their newer servers and workstations, and Tyan produces a dual-socket Opteron workstation or server motherboard that has 8 SAS ports in addition to the now-typical 6 SATA.