In Europe the acronym WLAN is widely used. It stands for Wireless LAN or Wireless Local Are Network. WLAN is not a trademark, so it can be used freely. On the other hand, it is easily confused with VLAN or Virtual LAN. VLAN is a wired LAN technology that can be used to separate traffic in the wired network.
Wi-Fi is a trademark owned by Wi-Fi Alliance. Wi-Fi Alliance defines standards and tests for interoperability between Wi-Fi products. Certified products will connect with each other. In practice all products are certified by Wi-Fi Alliance, although it does not have any official status. The term Wi-Fi is in wide use in the Americas, although the dash has started to disappear: WiFi.
802.11 is a technical standard for wireless networks defined by IEEE or Institute of Electrical and Electronics Engineers. After the original 802.11 there have been addendums and extensions, that have a character as a name: a, b etc. After z the naming was continued with aa, ab, ac, ad etc.