A thumb drive, also called a USB drive or flash drive, is a small solid-state drive that connects to a device through a USB port. Because USB technology has become such a standard protocol, users can transfer files to and from most personal computers easily with these small, portable drives.
Techopedia explains Thumb Drive
A USB drive is sometimes called a thumb drive because of its small size and shape. Thumb drives are generally less than a few inches long, with width and depth dimensions of fractions of an inch. These small drives typically hold at least 256 megabytes of data, with some models holding more than several gigabytes of data.
The solid state construction of a thumb drive means that unlike the hard disk drives common in larger devices, it has no moving parts. Data is stored through an integrated circuit design, allowing agile rewriting and dense data storage. The same kinds of technology have enabled manufacturers to produce other mini-items like camera storage cards, small mp3 players and much more.