Thumb drives are a subset of flash memory drives, which are small and can interface with another device through a USB port. Almost all thumb drives are flash drives, but not all flash-based storage drives are thumb drives.
Flash memory is a non-volatile, solid-state memory technology based on earlier reprogrammable read-only memory chips. When these chips were updated, they had to be completely erased and re-written with new information. This process was known as flashing the chip; this term eventually became part of the colloquial term for the modern storage technology. In addition to thumb drives, flash memory is used in memory cards and in solid-state hard drives.