WLAN is the acronym for Wireless Local Area Network, while WI-FI is the acronym for Wireless Fidelity. Both technologies are used in transferring data or files without the ancient method of data connection and transmission via cable. This means, no wire or cable is needed when Wi-Fi and WLAN are in use. WLAN is a type of network used by a computer which covers only a small area but gives a wireless means of transferring data.
Different computers within that place can be connected to a central system or computer, or other devices like scanner, printer, etc. Wireless LAN is based on IEEE 802.11as the standard for devices. Wi-Fi, on the other hand, is just a brand name for different types of devices that use wireless LAN. Wireless is also based on the same standard as Wireless LAN, i.e. IEEE 802.11. The simple thing is that any device using Wi-Fi is already using wireless LAN.