Pen drives, thumb drives, jump drives, keychain drives... they're called many things and they use surprisingly varied technologies, but they all share a few features in common: They're compact, removable drives that attach to your PC via a USB port, and add anywhere from 16 Mbyte to more than 4 Gbyte of portable storage to your system.
In operation, they act much like an ordinary floppy or hard drive. Many of these devices are even bootable, if the PC is of recent-enough vintage to allow booting from a USB device. (Check your BIOS settings for Boot or USB options.)
Newer operating systems (XP, Win2K, and some Linux distributions) automatically recognize and mount these USB drives, assigning the next-available drive letter. Older operating systems (e.g., Win9x) may require a driver to recognize the USB device.
There are three main types of compact, portable USB-drives. The smaller-capacity variants tend to be sealed units about the size of a normal adult thumb--hence the common name: thumb drive. (One specific brand of this kind of drive goes by the trade name "ThumbDrive," but we're using the term generically--as in "thumb-sized drive.") These units have no moving parts, and emulate the operation of a disk drive via solid-state electronics and memory chips that retain data even when the power is removed.
The midrange variants are generally two-piece units consisting of a "media reader" device into which you can insert postage-stamp-sized memory chips; the electronics in the reader let the PC access the memory chip as if it were a hard drive.
The upper end units use no emulation: They are sealed or semi-sealed units that contain an actual miniaturized hard drive (see this example) that's also only the size of a postage stamp.
There's considerable overlap among these groups in terms of features and capacities, but each type of device is aimed at a specialized purpose. Let's take a closer look, starting with thumb drives, whose features serve as a foundation for all three types of devices: