What is th difference between usb flash drive and usb pen drive?

What is th difference between usb flash drive and usb pen drive?

Nothing!
same entity.
There should be no difference, a USB drive is just a memory chip. No concern what form it comes in your computer will see it as an external thorny drive.
They're the same piece. See the various name in the wikipedia article.
Same article different name
No difference - both do equal thing - sometimes they are also call USB sticks...
None, really.

'flash' is the type of memory they use.