Serial Advanced Technology Attachment: connection bus between the motherboard and a mass storage device such as a hard drive.
The adoption of the serial mode was an evolution of the ATA bus, still known as IDE (Integrated Drive Electronics) and from now on as opposed to P-ATA, Parallel ATA.
The SATA standard was introduced in 2003, and made an increase in transfer speed possible. The theoretical transfer rate is 150MB/s and 300 MB/s for SATA version II, compared with 133MB/s for P-ATA with the Ultra DMA protocol.
Ironically, the previous parallel connection allowed higher transfer rates. But the logic was reversed by the increased speed at which the electronics run. On a series link, a series of bits can be sent at very high frequencies that a parallel link would not be able to handle.
The series mode has other advantages too: smaller cables and connectors and a longer maximum cable length.