Serial ATA (or SATA) is an interface - a means of moving data between a computer and storage devices.
SATA drives have much thinner cables and smaller connectors than Parallel ATA (or PATA) drives, which allow for more connectors on motherboard and better airflow inside the case. Furthermore the transfer-rate of SATA is much faster than PATA. The maximum length of SATA can be added to 1 m (eSATA can be up to 2m) while maximum length of Parallel ATA cable is only 0.45m.
The 1st generation SATA (also known as SATA 1.5Gb/s) is 150MB/s (or 1.5Gb/s). And the 2nd generation (also known as SATA 3.0Gb/s, backward compatible) is up to 300MB/s (or 3.0Gb/s). SATA 3.0Gb/s is also widely referred as SATA II (however SATA-IO declared that SATA II is an improper term).