The difference between digital and Wi-Fi-based baby monitors is that digital monitors send signals over what's known as the frequency hopping spread spectrum (FHSS). These send a signal directly and securely from a transmitter in the camera to the receiver on the parent unit.
Alternatively, Wi-Fi video baby monitors behave in the same way as all other devices currently connected to your Wi-Fi network – by connecting to your router. This does, of course, make it as susceptible to hackers as other devices on your network. As long as your network is protected and the camera's firmware is up to date, the risk is minimal.