WhatsApp Web is a computer based extension of the WhatsApp account on your phone. The messages you send and receive are fully synced between your phone and your computer, and you can see all messages on both devices. Any action you take on the phone will apply to WhatsApp Web and vice versa.
Comments
So what exactly is whatsapp web?
[edit] Found the answer at https://www.whatsapp.com/faq/en/web/28080003