Other contributors have said "What is the definition of pen drive?" is the same question as "What is a USB flash drive?". If you believe that these are not asking the same thing and should be answered differently, click here.
It is a memory card type device that can be plugged into the USB port on many PCs. It is very similar to a Floppy Drive, or a Disk Drive, but does not spin. It allows the user to transfer data (files, songs, images, etc) to/from computers quickly and easily.