WhatsApp has been a smartphone-only messaging program for ever, but that changes today. There’s now desktop clients for both Windows and OS X, in addition to the months-old web client.
This doesn’t reflect some giant change for WhatsApp — the desktop clients just mirror what’s on your phone. In other words, you need your phone powered on and running the WhatsApp app to send and receive messages. You scan a QR code on the desktop app with your smartphone, and what you see on your laptop is a reflection of the WhatsApp app on your phone, complete with sent messages and chats.