eSATA is a External SATA interface . Serial ATA is a commonly used interface for internal Hard Drives, and eSATA is an extension of that specification to be used with external devices.
Advantages of having eSATA port:
1.The transfer rate is fast. I mean really fast. Currently there is a 1.5 Gbps and a 3 Gbps Standard. This blows FireWire (IEEE 1394) and USB 2.0 out of the water.
2. Less Lag. Since the data is already in a native format, there is no translation involved. This means data gets to the interface faster.