An eSATA port is a port designed for use with an external SATA drive. In the case of your laptop model or desktop, this port can be used for a SATA device or as a USB port. The eSATA port is advantageous for use with an external drive because it allows for faster transfer speeds than a USB 2.0 port, and can be comparable, or better in some aspects, to the newer USB 3.0 speeds. These faster speeds are because the data does not have to be translated between the interface and the computer, but uses the same technologies as your internal hard drive.
eSATA provides faster transfer rates than SATA while also freeing up a USB port for some other duty when used with an external drive.
This eSATA port requires an additional power source if you do want to use it with an external drive but you can plug a USB drive straight into it because your laptop or desktop will provide the power it requires. An external drive needs additional power to operate. Look for one compatible with an eSATA port. So for an eSata port you would need a drive or a hard drive enclosure that allows for an alternate power source that plugs onto a power socket.
An eSATA port is usually a combination of eSATA and USB. That means any USB device plug will fit and work normally in it. Also note that eSATA ports on desktop PCs are usually unpowered. Laptops have eSATAp ports with the usual USB 5V voltage. There is also a third standard that adds 12V and can so power 3.5 inch drives
“The latest eSATA interface uses a new standard, the eSATA combo or eSATAp interface, and this is used to provide the speed of eSATA combined with power from a USB bus. This requires an eSATA combo port. These combo ports are typically found only on newer laptop computers and are generally designated by the word “eSATA” followed by a USB symbol as shown above.
Depending on how old your laptop or desktop is, the original eSATA specification does not provide power. You’ll need a USB cable or wall adapter to power the hard drive.
The easiest way to be sure if you have an eSATAp or eSata device or not is to check the connection. It should also indicate the type above the connection such as eSatap or eSata +.
This helpful image from Wikipedia shows the difference:
There are eSATA cables available, but if you are buying a hard drive to use with this, the cable could already be included. Also, keep in mind, that eSATA port can still be used as a standard USB 2.0 port, so you always have that option.