Written is a past participle of a word "Write."
Write- 1. Mark (letters, words, or other symbols) on a surface, typically paper, with a pen, pencil, or similar implement.
2. Compose, write, and send (a letter) to someone.
3. Compose (a text or work) for written or printed reproduction or publication; put into literary form and set down in writing.
4. Enter (data) into a specified storage medium or location in the store.
5. Underwrite (an insurance policy).
Poonam has written a letter to Prabh.