Wi-fi stands for Wireless fidelity,

The term Wi-Fi was created by the Wi-Fi Alliance, which oversees testing on a product, a product that passes the alliance tests is "Wi-Fi certified"

Wi-Fi is actually a trademark of the Wi-Fi Alliance that has certified products that belong to WLAN (wide local area network). So Wi-Fi actually certifies wireless routers

Yes, they are different although most people regard them as being one and the same. They are not. WLAN is an acronym for Wireless Local Area Network while Wi-Fi is described by the Wi-Fi Alliance as being any WLAN that is based upon IEEE 802.11 standards. To be Wi-Fi certified, the WLAN must successfully complete Wi-Fi Alliance interoperability certification testing. So, while any LAN that is Wi-Fi certified is regarded as a WLAN, not all WLAN are Wi-Fi certified.

Yes

It is to replaced WPA which requires testing and certification by Wi-Fi alliance. WPA2 is to provide a Wi-fi protection for users, a security technology.