The Wi-Fi network is protected by WPA2 encryption
A Wi-Fi signal is emitted in the form of a sphere (generally, as the actual form depends on the room in question) and can be accessed by all 'by design'. There are several methods of real-time encryption, but be careful, as some of them have known security loopholes that have been exploited.
Using WPA2 encryption is easy and provides a level of security that can be considered sufficient. Good management of access keys (complexity, duration, communication) is paramount.