Stands for "Serial Advanced Technology Attachment," or "Serial ATA." It is an interface used to connect ATA hard drives to a computer's motherboard. SATA transfer rates start at 150MBps, which is significantly faster than even the fastest 100MBps ATA/100 drives. For this and other reasons, Serial ATA is likely to replace the previous standard, Parallel ATA (PATA), which has been around since the 1980s.
Besides faster transfer rates, the SATA interface has several advantages over the PATA interface. For one, SATA drives each have their own independent bus, so there is no competition for bandwidth like there is with Parallel ATA. They also use smaller, thinner cables, which allows for better airflow inside the computer. SATA cables can be as long as one meter, while PATA cables max out at 40cm. This gives manufacturers more liberty when designing the internal layout of their computers. Finally, Serial ATA uses only 7 conductors, while Parallel ATA uses 40. This means there is less likely to be electromagnetic interference with SATA devices.
In summary, Serial ATA is a better, more efficient interface than the dated PATA standard. If you are looking to buy a computer that will support fast hard drives for years to come, make sure it comes with a SATA interface.