WiFi is a specific type of wireless networking technology based on the IEEE 802.11 standard. This standard is compatible with 802.3 Ethernet (the most common LAN type) since WiFi extends the edge of a wired network.
On the other hand, WLAN (Wireless Local Area Network) is a more general term that refers to any LAN that connects devices with radio technology instead of wiring. WiFi is one type of WLAN, but not the only one.
Read also Metered vs Unmetered Wifi.