http://en.wikipedia.org/wiki/Wi-Fi
Wi-Fi (also spelled Wifi or WiFi) is a popular technology that allows an electronic device to exchange data wirelessly (using radio waves) over a computer network, including high-speed Internet connections. The Wi-Fi Alliance defines Wi-Fi as any "wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards".[1] However, since most modern WLANs are based on these standards, the term "Wi-Fi" is used in general English as a synonym for "WLAN". Only Wi-Fi products that complete Wi-Fi Alliance interoperability certification testing successfully may use the "Wi-Fi CERTIFIED" trademark. ...

http://de.wikipedia.org/wiki/Wi-Fi
Wi-Fi (englische Aussprache: [ˈwaifai]) bezeichnet sowohl ein Firmenkonsortium, das Geräte mit Funkschnittstellen zertifiziert, als auch den zugehörigen Markenbegriff.
Wi-Fi ist ein für Marketingzwecke erfundener Kunstbegriff ohne (akronymische) Bedeutung, er steht nicht für „Wireless Fidelity“, wie man gemäß populärer Analogie z. B. zu Hi-Fi annehmen könnte. Allerdings wurde er von der Wi-Fi Alliance sehr wohl als Wortspiel zu „Hi-Fi“ aufgefasst.[1][2]
In vielen Ländern (USA, Großbritannien, Kanada, Spanien, Frankreich, Niederlande, Belgien, Italien, Republik Südafrika, Chile, Malta, Peru, Polen, Portugal, Russland, Bulgarien, Schweden, Uruguay u. a. sowie neuerdings auch Deutschland) wird Wi-Fi als Synonym für WLAN benutzt. Streng genommen sind WLAN und Wi-Fi jedoch nicht das gleiche, WLAN bezeichnet das Funknetzwerk, Wi-Fi hingegen den Funkstandard. In der Praxis kommen beide Begriffe aber nur gemeinsam vor, sodass sie in der Regel als Synonyme gebraucht werden. ...

WLAN is six syllables, WiFi is two, and sounds catchy. I've never heard anyone actually say WLAN in English (in ordinary speech).

Re: "I've never heard anyone actually say WLAN in English (in ordinary speech)"
Me neither, but I've heard "wireless LAN" and "W-LAN" (I think) in speech, albeit only very rarely.

As no me bre says, although they are not the same thing, they are in practice considered to be the same in informal use. I'm not actually aware of any other wireless LAN standards.

There are a number of wireless communication standards for notebook computers, smartphones and other devices, including WiMax, Bluetooth, others intended for devices (and televisions) in close proximity, etc.

No normal person here would ever say (and most would probably not even understand) WLAN for Wi-Fi (or WiFi).

If I want to connect to the internet, I might be interested in the availability of Wi-Fi, a certain way to get such a connection in many cases, or of Ethernet (another connection standard), both through a router which might or might not serve a local area network in addition to my connection. I don't usually care about the local network (LAN) as such, wired or wireless (except in my house and my office).

Wi-fi does not stand for wireless fidelity.
(Wireless Fidelity Debunked
The current confusion seems to stem from a brief period early in the days of the Wi-Fi Alliance when a regrettable tag line was added that stated, 'The Standard for Wireless Fidelity.' This was not part of the original name and was not created by Interbrand, but it was added as an afterthought in an attempt to help users make sense of the new and somewhat nonsensical word, Wi-Fi.)

#2: WLAN is six syllables, WiFi is two

In D spricht man es als We-Lan und Wi-Fi. Beides mit zwei Silben. Wai-Fai hört man seltener.