A portable hard drive refers to what is sometimes known as an external hard drive. Simply put, it is one that can be attached to a computer externally, by means of ports and cables. People have this type of device for uses such as data storage, like backups or simply as a second hard disk. Another potential use is for data transfers, such as taking information from a computer and moving it to another.
Unless stated differently, a portable hard drive will generally have a "traditional" hard disk inside of it. This means it is the kind that is running inside many PCs and laptops -- those who know the technicalities may understand that it has platters and an arm. In this sense, it has moving parts, and is not a solid state drive, which would be based upon something such as flash memory. Some people do prefer solid state technology for their data storage and transfer, particularly for external uses. For instance, a solid state drive, known as SSD for short, can be used in ways similar to a standard drive, but does not have any moving parts.
External Hard Drive
For people who want an even smaller device that also uses flash memory, a USB memory stick is an alternative to a portable hard drive. Ones publicly available do not yet hold terabytes (TB) of data, however, as of the time of writing, up to 128GB is a commonly available amount, although at rates per GB somewhat higher than lower-capacity models such as an 8GB or 16GB flash drive might cost.
Portable Hard Drive - What is This Kind of Hardware?