Wi-Fi is any wireless network that follows IEEE 802.11 specifications. It is ubiquitous and global. Provided you have not been hiding in a cave for the last decade, you know it well. Many IoT devices use it to connect to the internet, so it is useful to review how it works.

To connect to a Wi-Fi Local Area Network (WLAN), a device needs a network interface controller. This may be a separate controller card or simply integrated as part of the chipset inside the device. Communication uses Ethernet-style data packets over pre-identified radio communication bands. Packet delivery is not guaranteed and uses best effort delivery mechanism. Network protocol layers higher up in the stack, such as TCP or UDP, can offer guarantees of data delivery on top of the Wi-Fi standards.

IoT devices connect to a Wi-Fi access point that itself is (usually) connected to a wired Ethernet network that has internet access.

Wi-Fi offers very fast data rates but requires more power to support the constant back and forth communications. The universality of it makes it attractive for IoT devices. New standards such as 802.11ah are designed for low power scenarios. You should expect Wi-Fi to continue to be a heavily used connectivity option for IoT.