Although this drive supports ordinary USB 2.0, the main reason to buy it is for the eSATA port which seems to be roughly twice as fast at backing up data. If you don't know what eSATA is or don't see a need for it then buy the cheaper USB-only drive.
The slight downside of eSATA is that it needs to use two USB ports at a time; one for data and the other for power. The supplied cable is also pretty inflexible and the connector itself fussy about how cleanly it is plugged into the PC's eSATA port.
And, of course, your PC or laptop needs to have an eSATA connector - few PCs have this and only a few recent laptops do though it is possible to add them as an upgrade.
USB 3.0 drives will appear in the next few months, so it might be worth waiting for one of those. They will be expensive but probably about 2.5 times the speed of USB 2.0 (ignore the 10x speed claim as this will require SATA 3.0 hard drives which have yet to appear in any quantities).
Overall, a worthy product if you want more speed without spending a lot of money.