External Serial Advanced Technology Attachment (eSATA) is an extension to the Serial ATA standard that enables SATA drives to be attached externally.
Prior to eSATA, external hard drives were connected via USB 2.0 or FireWire. While eSATA can provide faster transfer rates than USB or FireWire, it requires its own power connector. Other terms commonly used for eSATA include eSATA/USB Combo and external SATA.
Recommended Reading: Understanding SCSI, ATA, SAS and SATA.