Flash drives have many names — jump drives, thumb drives, pen drives, and USB keychain drives. Regardless of what you call them, they all refer to the same thing, which is a small data storage device that uses flash memory and has a built-in USB connection.
Flash drives are typically no more than two to three inches in length and less than an inch in width. Their size and shape may resemble a thumb or a small pen (which is where the names "thumb drive" and "pen drive" come from). Flash drives are also very thin, often having a depth of less than a centimeter. Because of their small form factor, they are highly portable and can easily fit in a pocket or on a keychain (hence the name "keychain drive").
Early flash drives could store only a few megabytes of data, but modern flash drives can store several gigabytes of information. Since they are small in size but have large storage capacities, flash drives have replaced most previous portable data storage mediums such as floppy disks and removable hard disks like Zip disks. Because they have a built-in USB connection, flash drives also don't require a special disk drive to be used. Instead, they can be used on any computer with a USB port, which nearly all modern computers have.