On Facebook I see this scam advertising everyday, by clicking you can download Whatsapp for PC: how does it work?

The advertising is a scam: when you click it, a new windows will open and will ask to subscribe to a very expensive ringtone service. Once done that, we get the instructions page: the famous "Whatsapp per PC" it's just, an Android emulator.

In this way, you get a new emulated Android phone on your PC: you can chat with Whatsapp, but the app on your phone will be disabled! (fonte)

There is another way to get Whatsapp on a PC?

If your account is old, there is, it's Wassapp, it's an unofficial client made by Waninoko, an (ex) Wii hacker.

Do you know your username? It's your phone number; the password is the wifi MAC address if you use an iPhone or the IMEI code if you use  an Android smartphone. (If your account is new, the password is randomly generated)

Now, this is the main reason to stop using Whatsapp: the username is your phone number, everybody knows it. The password is the IMEI, easy to find by dialing *#06#. This means if you lend the phone to your friend, he could find your Whatsapp credentials and be able to read and send message in your name, easily, from his home PC!!!