What is a portable USB (2.0) hard drive ???
First of all, USB is an interface, that can be used for various computer peripherals, like a mouse, keyboard, printer, storage devices etc. Additionally, the USB interface is able to deliver some power (5V 500mA), which is quite enough for many devices. USB 2.0 is the newer, faster version of the original USB 1.1 specification.
Now for USB storage. Basically there are two types on the market. First are Flash (solid-state devices), usually in form of a "dongle" (called pendrive, stick). They store information in semiconductor memory. Second are hard disks, just like the ones installed in desktop and laptop computers - they store information on magnetic media.
Hard disks in a small enclosure, equipped with special interface, that enables them to be connected via the USB interface, are "portable USB hard drives". Many of them can be powered from the USB socket alone, some need an additional power supply.
With today's technology, hard disks can have 10-20 times larger capacity than flash devices, so they are still indispensable when you want to store a lot of big files (like pictures, movies etc), or back up the contents of your computer's hard drive as a safety copy.